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diff --git a/old/60990-0.txt b/old/60990-0.txt deleted file mode 100644 index b43c7c2..0000000 --- a/old/60990-0.txt +++ /dev/null @@ -1,9452 +0,0 @@ -*** START OF THE PROJECT GUTENBERG EBOOK 60990 *** - -[Illustration: - -_PLATE I_ -_Frontispiece_ - -GEM-STONES] - - - - - GEM-STONES - - AND THEIR DISTINCTIVE CHARACTERS - - - BY - - G. F. HERBERT SMITH - M.A., D.Sc. - OF THE BRITISH MUSEUM (NATURAL HISTORY) - - - WITH MANY DIAGRAMS AND THIRTY-TWO PLATES - OF WHICH THREE ARE IN COLOUR - - - THIRD EDITION - - - METHUEN & CO. LTD. - 36 ESSEX STREET W.C. - LONDON - - - _First Published_ _March 21st 1910_ - _Second Edition_ _June_ _1913_ - _Third Edition_ _1919_ - - - - - PREFACE - - -In this edition the opportunity has been taken to correct a few -misprints and mistakes that have been discovered in the first, and to -alter slightly one or two paragraphs, but otherwise no change has been -made. G. F. H. S. WANDSWORTH COMMON, S.W. - - - - - PREFACE TO THE FIRST EDITION - - -It has been my endeavour to provide in this book a concise, yet -sufficiently complete, account of the physical characters of the -mineral species which find service in jewellery, and of the methods -available for determining their principal physical constants to enable -a reader, even if previously unacquainted with the subject, to have -at hand all the information requisite for the sure identification of -any cut stone which may be met with. For several reasons I have dealt -somewhat more fully with the branches of science closely connected with -the properties of crystallized matter than has been customary hitherto -in even the most comprehensive books on precious stones. Recent years -have witnessed many changes in the jewellery world. Gem-stones are no -longer entirely drawn from a few well-marked mineral species, which -are, on the whole, easily distinguishable from one another, and it -becomes increasingly difficult for even the most experienced eye to -recognize a cut stone with unerring certainty. So long as the only -confusion lay between precious stones and paste imitations an ordinary -file was the solitary piece of apparatus required by the jeweller, but -now recourse must be had to more discriminative tests, such as the -refractive index or the specific gravity, the determination of which -calls for a little knowledge and skill. Concurrently, a keener interest -is being taken in the scientific aspect of gem-stones by the public at -large, who are attracted to them mainly by æsthetic considerations. - -While the treatment has been kept as simple as possible, technical -expressions, where necessary, have not been avoided, but their meanings -have been explained, and it is hoped that their use will not prove -stumbling-blocks to the novice. Unfamiliar words of this kind often -give a forbidding air to a new subject, but they are used merely to -avoid circumlocution, and not, like the incantations of a wizard, to -veil the difficulties in still deeper gloom. For actual practical work -the pages on the refractometer and its use and the method of heavy -liquids for the determination of specific gravities, and the tables of -physical constants at the end of the book, with occasional reference, -in case of doubt, to the descriptions of the several species alone -are required; other methods—such as the prismatic mode of measuring -refractive indices, or the hydrostatic way of finding specific -gravities—which find a place in the ordinary curriculum of a physics -course are described in their special application to gem-stones, but -they are not so suitable for workshop practice. Since the scope of the -book is confined mainly to the stones as they appear on the market, -little has been said about their geological occurrence; the case of -diamond, however, is of exceptional interest and has been more fully -treated. The weights stated for the historical diamonds are those -usually published, and are probably in many instances far from correct, -but they serve to give an idea of the sizes of the stones; the English -carat is the unit used, and the numbers must be increased by about 2½ -per cent. if the weights be expressed in metric carats. The prices -quoted for the various species must only be regarded as approximate, -since they may change from year to year, or even day to day, according -to the state of trade and the whim of fashion. - -The diagram on Plate II and most of the crystal drawings were made -by me. The remaining drawings are the work of Mr. H. H. Penton. He -likewise prepared the coloured drawings of cut stones which appear -on the three coloured plates, his models, with two exceptions, being -selected from the cut specimens in the Mineral Collection of the -British Museum by permission of the Trustees. Unfortunately, the -difficulties that still beset the reproduction of pictures in colour -have prevented full justice being done to the faithfulness of his -brush. I highly appreciate the interest he took in the work, and the -care and skill with which it was executed. My thanks are due to the -De Beers Consolidated Mines Co. Ltd., and to Sir Henry A. Miers, -F.R.S., Principal of the University of London, for the illustrations -of the Kimberley and Wesselton diamond mines, and of the methods and -apparatus employed in breaking up and concentrating the blue ground; -to Messrs. I. J. Asscher & Co. for the use of the photograph of the -Cullinan diamond; to Mr. J. H. Steward for the loan of the block of the -refractometer; and to Mr. H. W. Atkinson for the illustration of the -diamond-sorting machine. My colleague, Mr. W. Campbell Smith, B.A., has -most kindly read the proof-sheets, and has been of great assistance in -many ways. I hope that, thanks to his invaluable help, the errors in -the book which may have escaped notice will prove few in number and -unimportant in character. To Mr. Edward Hopkins I owe an especial debt -of gratitude for his cheerful readiness to assist me in any way in -his power. He read both the manuscript and the proof-sheets, and the -information with regard to the commercial and practical side of the -subject was very largely supplied by him. He also placed at my service -a large number of photographs, some of which—for instance, those -illustrating the cutting of stones—he had specially taken for me, and -he procured for me the jewellery designs shown on Plates IV and V. - -If this book be found by those engaged in the jewellery trade helpful -in their everyday work, and if it wakens in readers generally an -appreciation of the variety of beautiful minerals suitable for gems, -and an interest in the wondrous qualities of crystallized substances, I -shall be more than satisfied. - G. F. H. S. - WANDSWORTH COMMON, S.W. - - - - - CONTENTS - - - CHAP. PAGE - I. INTRODUCTION 1 - - - PART I—SECTION A - - THE CHARACTERS OF GEM-STONES - - II. CRYSTALLINE FORM 6 - - III. REFLECTION, REFRACTION, AND DISPERSION 14 - - IV. MEASUREMENT OF REFRACTIVE INDICES 21 - - V. LUSTRE AND SHEEN 37 - - VI. DOUBLE REFRACTION 40 - - VII. ABSORPTION EFFECTS: COLOUR, DICHROISM,ETC. 53 - - VIII. SPECIFIC GRAVITY 63 - - IX. HARDNESS AND CLEAVABILITY 78 - - X. ELECTRICAL CHARACTERS 82 - - - PART I—SECTION B - - THE TECHNOLOGY OF GEM-STONES - - XI. UNIT OF WEIGHT 84 - - XII. FASHIONING OF GEM-STONES 88 - - XIII. NOMENCLATURE OF PRECIOUS STONES 109 - - XIV. MANUFACTURED STONES 113 - - XV. IMITATION STONES 124 - - - PART II—SECTION A - - PRECIOUS STONES - - XVI. DIAMOND 128 - - XVII. OCCURRENCE OF DIAMOND 137 - - XVIII. HISTORICAL DIAMONDS 157 - - XIX. CORUNDUM (_Sapphire_, _Ruby_) 172 - - XX. BERYL (_Emerald_, _Aquamarine_, _Morganite_) 184 - - - PART II—SECTION B - - SEMI-PRECIOUS STONES - - XXI. TOPAZ 197 - - XXII. SPINEL (_Balas-Ruby_, _Rubicelle_) 203 - - XXIII. GARNET 207 - - (_a_) HESSONITE (_Grossular_, _Cinnamon-Stone_, - _Hyacinth_, _Jacinth_) 211 - - (_b_) PYROPE (‘_Cape-Ruby_’) 212 - - (_c_) RHODOLITE 214 - - (_d_) ALMANDINE (_Carbuncle_) 214 - - (_e_) SPESSARTITE 216 - - (_f_) ANDRADITE (_Demantoid_, _Topazolite_, ‘_Olivine_’) 216 - - (_g_) UVAROVITE 218 - - XXIV. TOURMALINE (_Rubellite_) 219 - - XXV. PERIDOT 225 - - XXVI. ZIRCON (_Jargoon_, _Hyacinth_, _Jacinth_) 228 - - XXVII. CHRYSOBERYL (_Chrysolite_, _Cat’s-Eye_, _Cymophane_, - _Alexandrite_) 233 - - XXVIII. QUARTZ (_Rock-Crystal_, _Amethyst_, _Citrine_, - _Cairngorm_, _Cat’s-Eye_, _Tiger’s-Eye_) 238 - - XXIX. CHALCEDONY, AGATE, ETC. 246 - - XXX. OPAL (_White Opal_, _Black Opal_, _Fire-Opal_) 249 - - XXXI. FELSPAR (_Moonstone_, _Sunstone_, _Labradorite_, - _Amazon-Stone_) 254 - - XXXII. TURQUOISE, ODONTOLITE, VARISCITE 257 - - XXXIII. JADE (NEPHRITE OR GREENSTONE, _Jadeite_) 260 - - XXXIV. SPODUMENE (_Kunzite_, _Hiddenite_), IOLITE, BENITOITE 265 - - XXXV. EUCLASE, PHENAKITE, BERYLLONITE 269 - - XXXVI. ENSTATITE (‘_Green Garnet_’), DIOPSIDE, KYANITE, - ANDALUSITE, IDOCRASE, EPIDOTE, SPHENE, AXINITE, - PREHNITE, APATITE, DIOPTASE 271 - - XXXVII. CASSITERITE, ANATASE, PYRITES, HEMATITE 281 - - XXXVIII. OBSIDIAN, MOLDAVITE 283 - - - PART II—SECTION C - - ORNAMENTAL STONES - - XXXIX. FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE, - AZURITE, MALACHITE, THULITE, MARBLE, APOPHYLLITE, - CHRYSOCOLLA, STEATITE OR SOAPSTONE, MEERSCHAUM, - SERPENTINE 285 - - - PART II—SECTION D - - ORGANIC PRODUCTS - - XL. PEARL, CORAL, AMBER 291 - - - TABLES - - I. CHEMICAL COMPOSITION OF GEM-STONES 300 - - II. COLOUR OF GEM-STONES 301 - - III. REFRACTIVE INDICES OF GEM-STONES 302 - - IV. COLOUR-DISPERSION OF GEM-STONES 303 - - V. CHARACTER OF THE REFRACTION OF GEM-STONES 303 - - VI. DICHROISM OF GEM-STONES 304 - - VII. SPECIFIC GRAVITIES OF GEM-STONES 305 - - VIII. DEGREES OF HARDNESS OF GEM-STONES 305 - - IX. DATA 306 - - - INDEX 307 - - - - - LIST OF PLATES - - - PAGE - I. GEM-STONES (in colour) _Frontispiece_ - - II. REFRACTIVE INDEX DIAGRAM 36 - - III. INTERFERENCE FIGURES 48 - - IV. JEWELLERY DESIGNS 62 - - V. JEWELLERY DESIGNS 88 - - VI. APPLIANCES USED FOR POLISHING DIAMONDS 102 - - VII. POLISHING DIAMONDS 103 - - VIII. SLITTING AND POLISHING COLOURED STONES 104 - - IX. FACETING MACHINE 105 - - X. LAPIDARY’S WORKSHOP AND OFFICE IN ENGLAND 106 - - XI. LAPIDARY’S WORKSHOP IN RUSSIA 107 - - XII. FRENCH FAMILY CUTTING STONES 108 - - XIII. INDIAN LAPIDARY 109 - - XIV. BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES - AND SAPPHIRES 118 - - XV. KIMBERLEY MINE, 1871 140 - - XVI. KIMBERLEY MINE, 1872 141 - - XVII. KIMBERLEY MINE, 1874 142 - - XVIII. KIMBERLEY MINE, 1881 143 - - XIX. KIMBERLEY MINE AT THE PRESENT DAY 144 - - XX. WESSELTON (open) MINE 145 - - XXI. LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT - OVER 146 - - XXII. WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND 147 - - XXIII. DIAMOND-SORTING MACHINES 148 - - XXIV. KAFIRS PICKING OUT DIAMONDS 149 - - XXV. CULLINAN DIAMOND (natural size) 168 - - XXVI. LARGE AQUAMARINE CRYSTAL (one-sixth natural size), - FOUND AT MARAMBAYA, MINAS GERAES, BRAZIL 196 - - XXVII. GEM-STONES (in colour) 226 - - XXVIII. OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES 252 - - XXIX. GEM-STONES (in colour) 256 - - XXX. NATIVES DRILLING PEARLS 294 - - XXXI. METAL FIGURES OF BUDDHA INSERTED IN A PEARL-OYSTER 296 - - XXXII. SECTIONS OF CULTURE PEARL 297 - - - - - GEM-STONES - - - - - CHAPTER I - - INTRODUCTION - - -Beauty, durability, and rarity: such are the three cardinal virtues -of a perfect gem-stone. Stones lacking any of them cannot aspire to -a high place in the ranks of precious stones, although it does not -necessarily follow that they are of no use for ornamental purposes. The -case of pearl, which, though not properly included among gem-stones, -being directly produced by living agency, yet holds an honoured place -in jewellery, constitutes to some extent an exception, since its -incontestable beauty atones for its comparative want of durability. - -That a gem-stone should be a delight to the eye is a truism that need -not be laboured; for such is its whole _raison d’être_. The members -of the Mineral Kingdom that find service in jewellery may be divided -into three groups, according as they are transparent, translucent, -or opaque. Of these the first, which is by far the largest and the -most important, may itself be further sub-divided into two sections: -stones which are devoid of colour, and stones which are tinted. Among -the former, diamond reigns supreme, since it alone possesses that -marvellous ‘fire,’ oscillating with every movement from heavenly blue -to glowing red, which is so highly esteemed and so much besought. -Other stones, such as ‘fired’ zircon, white sapphire, white topaz, and -rock-crystal, may dazzle with brilliancy of light reflected from the -surface or emitted from the interior, but none of them, like diamond, -glow with mysterious gleams. No hint of colour, save perhaps a trace -of the blue of steel, can be tolerated in stones of this category; -above all is a touch of the jaundice hue of yellow abhorred. It taxes -all the skill of the lapidary to assure that the disposition of the -facets be such as to reveal the full splendour of the stone. A coloured -stone, on the other hand, depends for its attractiveness more upon its -intrinsic hue than upon the manner of its cutting. The tint must not be -too light or too dark in shade: a stone that has barely any colour has -little interest, and one which is too dark appears almost opaque and -black. The lapidary can to some extent remedy these defects by cutting -the former deep and the latter shallow. In certain curious stones—for -instance tourmaline—the transparency, and in others—such as ruby, -sapphire, and one of the recent additions to the gem world, kunzite—the -colour, varies considerably in different directions. The colours that -are most admired—the fiery red of ruby, the royal blue of sapphire, -the verdant green of emerald, and the golden yellow of topaz—are pure -tints, and the absorption spectra corresponding to them are on the -whole continuous and often restricted. They therefore retain the purity -of their colour even in artificial light, though certain sapphires -transmit a relatively larger amount of red, and consequently turn -purple at night. Of the small group of translucent stones which pass -light, but are not clear enough to be seen through, the most important -is opal. It and certain others of the group owe their merit to the -same optical effect as that characterizing soap-bubbles, tarnished -steel, and so forth, and not to any intrinsic coloration. Another -set of stones—moonstone and the star-stones—reflect light from the -interior more or less regularly, but not in such a way as to produce -a play of colour. The last group, which comprises opaque stones, has -a single representative among ordinary gem-stones, namely, turquoise. -In this case light is scattered and reflected from layers immediately -contiguous to the surface, and the colour is due to the resulting -absorption. The apparent darkness of a deep-coloured stone follows from -a different cause: the light passing into the stone is wholly absorbed -within it, and, since none is emitted, the stone appears black. The -claims of turquoise are maintained by the blue variety; there is little -demand for stones of a greenish tinge. - -It is evidently desirable that any stones used in jewellery should be -able to resist the mechanical and chemical actions of everyday life. No -one is anxious to replace jewels every few years, and the most valuable -stones are expected to endure for all time. The mechanical abrasion -is caused by the minute grains of sand that are contained in ordinary -dust, and gem-stones should be at least as hard as they—a condition -fulfilled by all the principal species with the exception of opal, -turquoise, peridot, and demantoid. Since the beauty of the first named -does not depend on the brilliancy of its polish, scratches on the -surface are not of much importance; further, all four are only slightly -softer than sand. It may be noted that the softness of paste stones, -apart from any objections that may be felt to the use of imitations, -renders them unsuitable for jewellery purposes. The only stones that -are likely to be chemically affected in the course of wear are those -which are in the slightest degree porous. It is hazardous to immerse -turquoises in liquids, even in water, lest the bluish green colour be -oxidized to the despised yellowish hue. The risk of damage to opals, -moonstones, and star-stones by the penetration of dirt or grease into -the interior of the stones is less, but is not wholly negligible. -Similar remarks apply with even greater force to pearls. Their charm, -which is due to a peculiar surface-play of light, might be destroyed by -contamination with grease, ink, or similar matter; they are, moreover, -soft. For both reasons their use in rings is much to be deprecated. -Nothing can be more unsightly than the dingy appearance of a pearl ring -after a few years’ wear. - -It cannot be gainsaid that mankind prefers the rare to the beautiful, -and what is within reach of all is lightly esteemed. It is for this -reason that garnet and moonstone lie under a cloud. Purchasers can -readily be found for a ‘Cape-ruby’ or an ‘olivine,’ but not for a -garnet; garnets are so common, is the usual remark. Nevertheless, -the stones mentioned are really garnets. If science succeeded in -manufacturing diamonds at the cost of shillings instead of the pounds -that are now asked for Nature’s products—not that such a prospect is at -all probable or even feasible—we might expect them to vanish entirely -from fashionable jewellery. - -A careful study of the showcases of the most extensive jewellery -establishment brings to light the fact that, despite the apparent -profusion, the number of different species represented is restricted. -Diamond, ruby, emerald, sapphire, pearl, opal, turquoise, topaz, -amethyst are all that are ordinarily asked for. Yet, as later pages -will show, there are many others worthy of consideration; two among -them—peridot and tourmaline—are, indeed, slowly becoming known. For -the first five of the stones mentioned above, the demand is relatively -steady, and varies absolutely only with the purchasing power of the -world; but a lesser known stone may suddenly spring into prominence -owing to the caprice of fashion or the preference of some great lady -or leader of fashion. Not many years ago, for instance, violet was the -favourite colour for ladies’ dresses, and consequently amethysts were -much worn to match, but with the change of fashion they speedily sank -to their former obscurity. Another stone may perhaps figure at some -royal wedding; for a brief while it becomes the vogue, and afterwards -is seldom seen. - -Except that diamond, ruby, emerald, and sapphire, and, we should add, -pearl, may indisputably be considered to occupy the first rank, it is -impossible to form the gem-stones in any strict order. Every generation -sees some change. The value of a stone is after all merely what it will -fetch in the open market, and its artistic merits may be a matter of -opinion. The familiar aphorism, _de gustibus non est disputandum_, is a -warning not to enlarge upon this point. - - - - - PART I—SECTION A - - THE CHARACTERS OF GEM-STONES - - - - - CHAPTER II - - CRYSTALLINE FORM - - -With the single exception of opal, the whole of the principal mineral -species used in jewellery are distinguished from glass and similar -substances by one fundamental difference: they are crystallized matter, -and the atoms composing them are regularly arranged throughout the -structure. - -The words crystal and glass are employed in science in senses differing -considerably from those in popular use. The former of them is derived -from the Greek word κρύος, meaning ice, and was at one time used in -that sense. For instance, the old fourteenth-century reading of Psalm -cxlvii. 17, which appears in the authorized version as “He giveth his -ice like morsels,” ran “He sendis his kristall as morcels.” It was -also applied to the beautiful, lustrous quartz found among the eternal -snows of the Alps, since, on account of their limpidity, these stones -were supposed, as Pliny tells us, to consist of water congealed by the -extreme cold of those regions; such at the present day is the ordinary -meaning of the word. But, when early investigators discovered that a -salt solution on evaporation left behind groups of slender glistening -prisms, each very similar to the rest, they naturally—though, as we -now know, wrongly—regarded them as representing yet another form -of congealed water, and applied the same word to such substances. -Subsequent research has shown that these salts, as well as mineral -substances occurring with natural faces in nature, have in common the -fundamental property of regularity of arrangement of the constituent -atoms, and science therefore defines by the word crystal a substance in -which the structure is uniform throughout, and all the similar atoms -composing it are arranged with regard to the structure in a similar way. - -The other word is yet more familiar; it denotes the transparent, -lustrous, hard, and brittle substance produced by the fusion of sand -with soda or potash or both which fills our windows and serves a -variety of useful purposes. Research has shown that glass, though -apparently so uniform in character, has in reality no regularity of -molecular arrangement. It is, in fact, a kind of mosaic of atoms, -huddled together anyhow, but so irregular is its irregularity that -it simulates perfect regularity. Science uses the word glass in this -widened meaning. Two substances may, as a matter of fact, have the -same chemical composition, and one be a crystal and the other a glass. -For example, quartz, if heated to a high temperature, may be fused and -converted into a glass. The difference in the two types of structure -may be illustrated by a comparison between a regiment of soldiers -drawn up on parade and an ordinary crowd of people. - -The crystalline form is a visible sign of the molecular arrangement, -and is intimately associated with the directional physical properties, -such as the optical characters, cleavage, etc. A study of it is not -only of interest in itself, but also of great importance to the -lapidary who wishes to cut a stone to the best advantage, and it is, -moreover, of service in distinguishing stones when in the rough state. - -[Illustration: FIG. 1.—Cubo-Octahedra.] - -The development of natural faces on a crystal is far from being -haphazard, but is governed by the condition that the angles between -similar faces, whether on the same crystal or on different crystals, -are equal, however varying may be the shapes and the relative sizes -of the faces (Fig. 1), and by the tendency of the faces bounding -the crystal to fall into series with parallel edges, such series -being termed zones. Each species has a characteristic type of -crystallization, which may be referred to one of the following six -systems:— - -1. _Cubic._—Crystals in this system can be referred to three edges, -which are mutually at right angles, and in which the directional -characters are identical in value. These principal edges are known -as axes. Some typical forms are the cube (Fig. 2), characteristic -of fluor; the octahedron (Fig. 3), characteristic of diamond and -spinel; the dodecahedron (Fig. 4), characteristic of garnet; and the -triakisoctahedron, or three-faced octahedron (Fig. 5). - -[Illustration: FIG. 2.—Cube.] - -[Illustration: FIG. 3.—Octahedron.] - -[Illustration: FIG. 4.—Dodecahedron.] - -All crystals belonging to this system are singly refractive. - -2. _Tetragonal._—Such crystals can be referred to three axes, which are -mutually at right angles, but in only two of them are the directional -characters identical. A typical form is a four-sided prism, _mm_, of -square section, terminated by four triangular faces, _p_ (Fig. 6), the -usual shape of crystals of zircon and idocrase. - -[Illustration: FIG. 5.—Triakisoctahedron, or Three-faced Octahedron.] - -[Illustration: FIG. 6.—Tetragonal Crystal.] - -Crystals belonging to this system are doubly refractive and uniaxial, -_i.e._ they have one direction of single refraction (cf. p. 45), which -is parallel to the unequal edge of the three mentioned above. - -[Illustration: FIG. 7.—Two alternative sets of Axes in the Hexagonal -System.] - -3. _Hexagonal._—Such crystals can be referred alternatively either -to a set of three axes, _X_, _Y_, _Z_ (Fig. 7), which lie in a plane -perpendicular to a fourth, _H_, and are mutually inclined at angles of -60°, or to a set of three, _a_, _b_, _c_, which are not at right angles -as in the cubic system, but in which the directional characters are -identical. The fourth axis in the first arrangement is equally inclined -to each in the second set of axes. Many important species crystallize -in this system—corundum (sapphire, ruby), beryl (emerald, aquamarine), -tourmaline, quartz, and phenakite. The crystals usually display a -six-sided prism, terminated by a single face, _c_, perpendicular to the -edge of the prism _m_ (Fig. 8), _e.g._ emerald, or by six or twelve -inclined faces, _p_ (Fig. 9), _e.g._ quartz, crystals of which are so -constant in form as to be the most familiar in the Mineral Kingdom. -Tourmaline crystals (Fig. 10) are peculiar because of the fact that -often one end is obviously to the eye flatter than the other. - -[Illustration: FIGS. 8-10.—Hexagonal Crystals.] - -Crystals belonging to this system are also doubly refractive and -uniaxial, the direction of single refraction being parallel to the -fourth axis mentioned above, and therefore also parallel to the prism -edge. Hence deeply coloured tourmaline, which strongly absorbs the -ordinary ray, must be cut with the table-facet parallel to the edge of -the prism. - -[Illustration: FIG. 11.—Relation of the two directions of single -Refraction to the Axes in an Orthorhombic Crystal.] - -4. _Orthorhombic._—Such crystals can be referred to three axes, which -are mutually at right angles, but in which each of the directional -characters are different. The crystals are usually prismatic in shape, -one of the axes being parallel to the prism edge. Topaz, peridot, and -chrysoberyl are the most important species crystallizing in this system. - -Crystals belonging to this system are doubly refractive and biaxial, -_i.e._ they have two directions of single refraction (cf. p. 45). The -three axes _a_, _b_, _c_ (Fig. 11) are parallel respectively to the two -bisectrices of the directions of single refraction, and the direction -perpendicular to the plane containing those directions. - -5. _Monoclinic._—Such crystals can be referred to three axes, one -of which is at right angles to the other two, which are, however, -themselves not at right angles. Spodumene (kunzite) and some moonstone -crystallize in this system. - -Crystals belonging to this system are doubly refractive and biaxial, -but in this case the first axis alone is parallel to one of the -principal optical directions. - -6. _Triclinic._—Such crystals have no edges at right angles, and the -optical characters are not immediately related to the crystalline form. -Some moonstone crystallizes in this system. - -[Illustration: FIG. 12.—Twinned Octahedron.] - -Crystals are often not single separate individuals. For instance, -diamond and spinel are found in flat triangular crystals with their -girdles cleft at the corners (Fig. 12). Each of such crystals is -really composed of portions of two similar octahedra, which are placed -together in such a way that each is a reflection of the other. Such -composite crystals are called twins or macles. Sometimes the twinning -is repeated, and the individuals may be so minute as to call for a -microscope for their perception. - -A composite structure may also result from the conjunction of -numberless minute individuals without any definite orientation, as in -the case of chalcedony and agate. So by supposing the individuals to -become infinitesimally small, we pass to a glass-like substance. - -It is often a peculiarity of crystals of a species to display a typical -combination of natural faces. Such a combination is known as the habit -of the species, and is often of service for the purpose of identifying -stones before they are cut. Thus, a habit of diamond and spinel is -an octahedron, often twinned, of garnet a dodecahedron, of emerald a -flat-ended hexagonal prism, and so on. - -It is one of the most interesting and remarkable features connected -with crystallization that the composition and the physical -characters—for instance, the refractive indices and specific -gravity—may, without any serious disturbance of the molecular -arrangement, vary considerably owing to the more or less complete -replacement of one element by another closely allied to it. That is -the cause of the range of the physical characters which has been -observed in such species as tourmaline, peridot, spinel, etc. The -principal replacements in the case of the gem-stones are ferric oxide, -Fe_{2}O_{3}, by alumina, Al_{2}O_{3}, and ferrous oxide, FeO, by -magnesia, MgO. - -A list of the principal gem-stones, arranged by their chemical -composition, is given in Table I at the end of the book. - - - - - CHAPTER III - - REFLECTION, REFRACTION, AND DISPERSION - - -It is obvious that, since a stone suitable for ornamental use must -appeal to the eye, its most important characters are those which depend -upon light; indeed, the whole art of the lapidary consists in shaping -it in such a way as to show these qualities to the best advantage. To -understand why certain forms are given to a cut stone, it is essential -for us to ascertain what becomes of the light which falls upon the -surface of the stone; further, we shall find that the action of a -stone upon light is of very great help in distinguishing the different -species of gem-stones. The phenomena displayed by light which impinges -upon the surface separating two media[1] are very similar in character, -whatever be the nature of the media. - -Ordinary experience with a plane mirror tells us that, when light is -returned, or reflected, as it is usually termed, from a plane or flat -surface, there is no alteration in the size of objects viewed in this -way, but that the right and the left hands are interchanged: our right -hand becomes the left hand in our reflection in the mirror. We notice, -further, that our reflection is apparently just as far distant from the -mirror on the farther side as we are on this side. In Fig. 13 _MM´_ -is a section of the mirror, and _O´_ is the image of the hand _O_ as -seen in the mirror. Light from _O_ reaches the eye _E_ by way of _m_, -but it appears to come from _O´_. Since _OO´_ is perpendicular to the -mirror, and _O_ and _O´_ lie at equal distances from it, it follows -from elementary geometry that the angle _i´_, which the reflected ray -makes with _mn_, the normal to the mirror, is equal to _i_, the angle -which the incident ray makes with the same direction. - -[Illustration: FIG. 13.—Reflection at a Plane Mirror.] - -Again, everyday experience tells us that the case is less simple when -light actually crosses the bounding surface and passes into the other -medium. Thus, if we look down into a bath filled with water, the -bottom of the bath appears to have been raised up, and a stick plunged -into the water seems to be bent just at the surface, except in the -particular case when it is perfectly upright. Since the stick itself -has not been bent, light evidently suffers some change in direction as -it passes into the water or emerges therefrom. The passage of light -from one medium to another was studied by Snell in the seventeenth -century, and he enunciated the following laws:— - -1. The refracted ray lies in the plane containing the incident ray and -the normal to the plane surface separating the two media. - -It will be noticed that the reflected ray obeys this law also. - -2. The angle _r_, which the refracted ray makes with the normal, is -related to the angle _i_, which the incident ray makes with the same -direction, by the equation - - _n_ sin _i_ = _n´_ sin _r_, (_a_) - -where _n_ and _n´_ are constants for the two media which are known as -the indices of refraction, or the refractive indices. - -This simple trigonometrical relation may be expressed in geometrical -language. Suppose we cut a plane section through the two media at right -angles to the bounding plane, which then appears as a straight line, -_SOS´_ (Fig. 14), and suppose that _IO_ represents the direction of -the incident ray; then Snell’s first law tells us that the refracted -ray _OR_ will also lie in this plane. Draw the normal _NON´_, and with -centre _O_ and any radius describe a circle intersecting the incident -and refracted rays in the points _a_ and _b_ respectively; let drop -perpendiculars _ac_ and _bd_ on to the normal _NON´_. Then we have -_n.ac = n´.bd_, whence we see that if _n_ be greater than _n´_, _ac_ -is less than _bd_, and therefore when light passes from one medium -into another which is less optically dense, in its passage across the -boundary it is bent, or refracted, away from the normal. - -[Illustration: FIG. 14.—Refraction across a Plane Surface.] - -We see, then, that when light falls on the boundary of two different -media, some is reflected in the first and some is refracted into the -second medium. The relative amounts of light reflected and refracted -depend on the angle of incidence and the refractive indices of the -media. We shall return to this point when we come to consider the -lustre of stones. - -We will proceed to consider the course of rays at different angles of -incidence when light passes out from a medium into one less dense—for -instance, from water into air. Corresponding to light with a small -angle of incidence such as _I_{1}O_ (Fig. 15), some of it is reflected -in the direction _OI´_{1}_ and the remainder is refracted out in the -direction _OR_{1}_. Similarly, for the ray _I_{2}O_ some is reflected -along _OI´_{2}_ and some refracted along _OR_{2}_. Since, in the case -we have taken, the angle of refraction is greater than the angle of -incidence, the refracted ray corresponding to some incident, ray -_I_{c}O_ will graze the bounding surface, and corresponding to a -ray beyond it, such as _I_{3}O_, which has a still greater angle of -incidence, there is no refracted ray, and all the light is wholly -or totally reflected within the dense medium. The critical angle -_I_{c}ON_, which is called the angle of total-reflection, is very -simply related to the refractive indices of the two media; for, since -_r_ is now a right angle, sin _r_ = 1, and equation (_a_) becomes - - _n_ sin _i_ = _n´_ (_b_) - -Hence, if the angle of total-reflection is measured and one of the -indices is known, the other can easily be calculated. - -[Illustration: FIG. 15.—Total-Reflection.] - -The phenomenon of total-reflection may be appreciated if we hold a -glass of water above our head, and view the light of a lamp on a table -reflected from the under surface of the water. This reflection is -incomparably more brilliant than that given by the upper surface. - -The refractive index of air is taken as unity; strictly, it is that of -a vacuum, but the difference is too small to be appreciated even in -very delicate work. Every substance has different indices for light -of different colour, and it is customary to take as the standard the -yellow light of a sodium flame. This happens to be the colour to -which our eyes are most sensitive, and a flame of this kind is easily -prepared by volatilizing a little bicarbonate of soda in the flame of -a bunsen burner. A survey of Table III at the end of the book shows -clearly how valuable a measurement of the refractive index is for -determining the species to which a cut stone belongs. The values found -for different specimens of the species do in cases vary considerably -owing to the great latitude possible in the chemical constitution -due to the isomorphous replacement of one element by another. Some -variation in the index may even occur in different directions within -the same stone; it results from the remarkable property of splitting up -a beam of light into two beams, which is possessed by many crystallized -substances. This forms the subject of a later chapter. - -Upon the fact that the refractive index of a substance varies for -light of different colours depends such familiar phenomena as the -splendour of the rainbow and the ‘fire’ of the diamond. When white -light is refracted into a stone it no longer remains white, but is -split up into a spectrum. Except in certain anomalous substances the -refractive index increases progressively as the wave-length of the -light decreases, and consequently a normal spectrum is violet at one -end and passes through green and yellow to red at the other end. The -width of the spectrum, which may be measured by the difference between -the refractive indices for the extreme red and violet rays, also -varies, though on the whole it increases with the refractive index. It -is the dispersion, as this difference is termed, that determines the -‘fire’—a character of the utmost importance in colourless transparent -stones, which, but for it, would be lacking in interest. Diamond excels -all colourless stones in this respect, although it is closely followed -by zircon, the colour of which has been driven off by heating; it is, -however, surpassed by two coloured species: sphene, which is seldom -seen in jewellery, and demantoid, the green garnet from the Urals, -which often passes under the misnomer ‘olivine.’ The dispersion of the -more prominent species for the _B_ and _G_ lines of the solar spectrum -is given in Table IV at the end of the book. - -We will now proceed to discuss methods that may be used for the -measurement of the refractive indices of cut stones. - - - - - CHAPTER IV - - MEASUREMENT OF REFRACTIVE INDICES - - -The methods available for the measurement of refractive indices are of -two kinds, and make use of two different principles. The first, which -is based upon the very simple relation found in the last chapter to -subsist at total-reflection, can be used with ease and celerity, and is -best suited for discriminative purposes; but it is restricted in its -application. The second, which depends on the measurement of the angle -between two facets and the minimum deviation experienced by a ray of -light when traversing a prism formed by them, is more involved, entails -the use of more elaborate apparatus, and takes considerable time, but -it is less restricted in its application. - - - (1) THE METHOD OF TOTAL-REFLECTION - -We see from equation _b_ (p. 18), connecting the angle of -total-reflection with the refractive indices of the adjacent media, -that, if the denser medium be constant, the indices of all less dense -media may be easily determined from a measurement of the corresponding -critical angle. In all refractometers the constant medium is a glass -with a high refractive index. Some instruments have rotatory parts, by -means of which this angle is actually measured. Such instruments give -very good results, but suffer from the disadvantages of being neither -portable nor convenient to handle, and of not giving a result without -some computation. - -[Illustration: FIG. 16.—Refractometer (actual size).] - -For use in the identification of cut stones, a refractometer with a -fixed scale, such as that (Fig. 16) devised by the author, is far -more convenient. In order to facilitate the observations, a totally -reflecting prism has been inserted between the two lenses of the -eyepiece. The eyepiece may be adjusted to suit the individual eyesight; -but for observers with exceptionally long sight an adapter is provided, -which permits the eyepiece being drawn out to the requisite extent. -The refractometer must be held in the manner illustrated in Fig. -17, so that the light from a window or other source of illumination -enters the instrument by the lenticular opening underneath. Good, -even illumination of the field may also very simply be secured by -reflecting light into the instrument from a sheet of white paper laid -on a table. On looking down the eyepiece we see a scale (Fig. 18), the -eyepiece being, if necessary, focused until the divisions of the scale -are clearly and distinctly seen. Suppose, for experiment, we smear a -little vaseline or similar fatty substance on the plane surface of the -dense glass, which just projects beyond the level of the brass plate -embracing it. The field of view is now no longer uniformly illuminated, -but is divided into two parts (Fig. 19): a dark portion above, which -terminates in a curved edge, apparently green in colour, and a bright -portion underneath, which is composed of totally reflected light. If -we tilt the instrument downwards so that light enters the instrument -from above through the vaseline we find that the portions of the field -are reversed, the dark portion being underneath and terminated by -a red edge. It is possible so to arrange the illumination that the -two portions are evenly lighted, and the common edge becomes almost -invisible. It is therefore essential for obtaining satisfactory results -that the plate and the dense glass be shielded from the light by the -disengaged hand. The shadow-edge is curved, and is, indeed, an arc of a -circle, because spherical surfaces are used in the optical arrangements -of the refractometer; by the substitution of cylindrical surfaces it -becomes straight, but sufficient advantage is not secured thereby to -compensate for the greatly increased complexity of the construction. -The shadow-edge is coloured, because the relative dispersion, - _n{v}_ − _n{r}_ - ——————————————— (_n{v}_ and _n{r}_ being the refractive indices for - _n_ -the extreme violet and red rays respectively), of the vaseline differs -from that of the dense glass. The dispersion of the glass is very -high, and exceeds that of any stone for which it can be used. Certain -oils have, however, nearly the same relative dispersion, and the edges -corresponding to them are consequently almost colourless. A careful eye -will perceive that the coloured shadow-edge is in reality a spectrum, -of which the violet end lies in the dark portion of the field and the -red edge merges into the bright portion. The yellow colour of a sodium -flame, which, as has already been stated, is selected as the standard -for the measurement of refractive indices, lies between the green and -the red, and the part of the spectrum to be noted is at the bottom of -the green, and practically, therefore, at the bottom of the shadow, -because the yellow and red are almost lost in the brightness of the -lower portion of the field. If a sodium flame be used as the source of -illumination, the shadow-edge becomes a sharply defined line. The scale -is so graduated and arranged that the reading where this line crosses -the scale gives the corresponding refractive index, the reading, since -the line is curved, being taken in the middle of the field on the -right-hand side of the scale. The refractometer therefore gives at -once, without any intermediate calculation, a value of the refractive -index to the second place of decimals, and a skilled observer -may, by estimating the tenths of the intervals between successive -divisions, arrive at the third place; to facilitate this estimation -the semi-divisions beyond 1·650 have been inserted. The range extends -nearly to 1·800; for any substance with a higher refractive index the -field is dark as far as the limit at the bottom. - -[Illustration: FIG. 17.—Method of Using the Refractometer.] - -[Illustration: FIG. 18.—Scale of the Refractometer.] - -[Illustration: FIG. 19.—Shadow-edge given by a singly refractive -Substance.] - -A fat, or a liquid, wets the glass, _i.e._ comes into intimate contact -with it, but if a solid substance be tested in the same way, a film of -air would intervene and entirely prevent an observation. To displace -it, a drop of some liquid which is more highly refractive than the -substance under test must first be applied to the plane surface -of the dense glass. The most convenient liquid for the purpose is -methylene iodide, CH_{2}I_{2}, which, when pure, has at ordinary room -temperatures a refractive index of 1·742. It is almost colourless when -fresh, but turns reddish brown on exposure to light. If desired, it -may be cleared in the manner described below (p. 66), but the film -of liquid actually used is so thin that this precaution is scarcely -necessary. If we test a piece of ordinary glass—one of the slips used -by microscopists for covering thin sections is very convenient for -the purpose—first applying a drop of methylene iodide to the plane -surface of the dense glass of the refractometer (Fig. 20), we notice a -coloured shadow-edge corresponding to the glass-slip at about 1·530 and -another, almost colourless, at 1·742, which corresponds to the liquid. -If the solid substance which is tested is more highly refractive than -methylene iodide, only the latter of the shadow-edges is visible, and -we must utilize some more refractive liquid. We can, however, raise -the refractive index of methylene iodide by dissolving sulphur[2] in -it; the refractive index of the saturated liquid lies well beyond -1·800 and the shadow-edge corresponding to it, therefore, does not -come within the range of the refractometer. The pure and the saturated -liquids can be procured with the instrument, the bottles containing -them being japanned on the outside to exclude light and fitted with -dipping-stoppers, by means of which a drop of the liquid required is -easily transferred to the surface of the glass of the instrument. -So long as the liquid is more highly refractive than the stone, or -whatever may be the substance under examination, its precise refractive -index is of no consequence. The facet used in the test must be flat, -and must be pressed firmly on the instrument, so that it is truly -parallel to the plane surface of the dense glass; for good results, -moreover, it must be bright. - -[Illustration: FIG. 20.—Faceted Stone in Position on the Refractometer.] - -[Illustration: FIG. 21.—Shadow-edges given by a doubly refractive -substance.] - -We have so far assumed that the substance which we are testing is -simple and gives a single shadow-edge; but, as may be seen from Table -V, many of the gem-stones are doubly refractive, and such will, -in general, show in the field of the refractometer two distinct -shadow-edges more or less widely separated. Suppose, for example, we -study the effect produced by a peridot, which displays the phenomenon -to a marked degree. If we revolve the stone so that the facet under -observation remains parallel to the plane surface of the dense glass -of the refractometer and in contact with it, we notice that both the -shadow-edges in general move up or down the scale. In particular cases, -depending upon the relation of the position of the facet selected to -the crystalline symmetry, one or both of them may remain fixed, or -one may even move across the other. But whatever facet of the stone -be used for the test, and however variable be the movements of the -shadow-edges, the highest and lowest readings obtainable remain the -same; they are the principal indices of refraction, such as are stated -in Table III at the end of the book, and their difference measures -the maximum amount of double refraction possessed by the stone. The -procedure is therefore simplicity itself; we have merely to revolve -the stone on the instrument, usually through not more than a right -angle, and note the greatest and least readings. It will be noticed -that the shadow-edges cross the scale symmetrically in the critical and -skewwise in intermediate positions. Fig. 21 represents the effect when -the facet is such as to give simultaneously the two readings required. -The shadow-edges _a_ and _b_, which are coloured in white light, -correspond to the least and greatest respectively of the principal -refractive indices, while the third shadow-edge, which is very faint, -corresponds to the liquid used—methylene iodide. It is possible, as we -shall see in a later chapter, to learn from the motion, if any, of the -shadow-edges something as to the character of the double refraction. -Since, however, each shadow-edge is spectral in white light, they will -not be distinctly separate unless the double refraction exceeds the -relative dispersion. Topaz, for instance, appears in white light to -yield only a single shadow-edge, and may thus easily be distinguished -from tourmaline, in which the double refraction is large enough for -the separation of the two shadow-edges to be clearly discerned. In -sodium light, however, no difficulty is experienced in distinguishing -both the shadow-edges given by substances with small amount of double -refraction, such as chrysoberyl, quartz, and topaz, and a skilled -observer may detect the separation in the extreme instances of apatite, -idocrase, and beryl. The shadow-edge corresponding to the greater -refractive index is always less distinct, because it lies in the bright -portion of the field. If the stone or its facet be small, it must -be moved on the plane surface of the dense glass until the greatest -possible distinctness is imparted to the edge or edges. If it be moved -towards the observer from the further end, a misty shadow appears to -move down the scale until the correct position is reached, when the -edges spring into view. - -Any facet of a stone may be utilized so long as it is flat, but the -table-facet is the most convenient, because it is usually the largest, -and it is available even when the stone is mounted. That the stone need -not be removed from its setting is one of the great advantages of this -method. The smaller the stone the more difficult it is to manipulate; -caution especially must be exercised that it be not tilted, not only -because the shadow-edge would be shifted from its true position and an -erroneous value of the refractive index obtained, but also because a -corner or edge of the stone would inevitably scratch the glass of the -instrument, which is far softer than the hard gem-stones. Methylene -iodide will in time attack and stain the glass, and must therefore be -wiped off the instrument immediately after use. - - - (2) THE METHOD OF MINIMUM DEVIATION - -If the stone be too highly refractive for a measurement of its -refractive index to be possible with the refractometer just described, -and it is desired to determine this constant, recourse must be had -to the prismatic method, for which purpose an instrument known as -a goniometer[3] is required. Two angles must be measured; one the -interior angle included between a suitable pair of facets, and the -other the minimum amount of the deviation produced by the pair upon a -beam of light traversing them. - -[Illustration: FIG. 22.—Path at Minimum Deviation of a Ray traversing a -Prism formed of two Facets of a Cut Stone.] - -Fig. 22 represents a section of a step-cut stone perpendicular to a -series of facets with parallel edges; _t_ is the table, and _a, b, c_, -are facets on the culet side. The path of light traversing the prism -formed by the pair of facets, _t_ and _b_, is indicated. Suppose that -_A_ is the interior angle of the prism, _i_ the angle of incidence of -light at the first facet and _i´_ the angle of emergence at the second -facet, and _r_ and _r´_ the angles inside the stone at the two facets -respectively. Then at the first facet light has been bent through an -angle _i - r_, and again at the second facet through an angle _i´ - -r´_; the angle of deviation, _D_, is therefore given by - - _D = i + i´ - (r + r´)_. - -We have further that - - _r + r´ = A_, - -whence it follows that - - _A + D = i + i´._ - -If the stone be mounted on the goniometer and adjusted so that the -edge of the prism is parallel to the axis of rotation of the instrument -and if light from the collimator fall upon the table-facet and the -telescope be turned to the proper position to receive the emergent -beam, a spectral image of the object-slit, or in the case of a doubly -refractive stone in general, two spectral images, will be seen in -white light; in the light of a sodium flame the images will be sharp -and distinct. Suppose that we rotate the stone in the direction of -diminishing deviation and simultaneously the telescope so as to retain -an image in the field of view, we find that the image moves up to and -then away from a certain position, at which, therefore, the deviation -is a minimum. The image moves in the same direction from this position -whichever way the stone be rotated. The question then arises what are -the angles of incidence and refraction under these special conditions. -It is clear that a path of light is reversible; that is to say, if a -beam of light traverses the prism from the facet _t_ to the facet _b_ -it can take precisely the same path from the facet _b_ to the facet -_t_. Hence we should be led to expect that, since experiment teaches us -that there is only one position of minimum deviation corresponding to -the same pair of facets, the angles at the two facets must be equal, -_i.e._ _i = i´_, and _r = r´_. It is, indeed, not difficult to prove by -either geometrical or analytical methods that such is the case. - - _A_ _A_ + _D_ -Therefore at minimum deviation _r_ = ——— and _i_ = —————————, and, - 2 2 -since sin _i_ = _n_ sin _r_, where _n_ is the refractive index of -the stone, we have the simple relation— - _A_ + _D_ - sin ————————— - 2 - _n_ = ————————————— - _A_ - sin ——— - 2 - -This relation is strictly true only when the direction of minimum -deviation is one of crystalline symmetry in the stone, and holds -therefore in general for all singly refractive stones, and for the -ordinary ray of a uniaxial stone; but the values thus obtained even in -the case of biaxial stones are approximate enough for discriminative -purposes. If then the stone be singly refractive, the result is the -index required; if it be uniaxial, one value is the ordinary index -and the other image gives a value lying between the ordinary and the -extraordinary indices; if it be biaxial, the values given by the two -images may lie anywhere between the greatest and the least refractive -indices. The angle _A_ must not be too large; otherwise the light will -not emerge at the second facet, but will be totally reflected inside -the stone: on the other hand, it must not be too small, because any -error in its determination would then seriously affect the accuracy of -the value derived for the refractive index. Although the monochromatic -light of a sodium flame is essential for precise work, a sufficiently -approximate value for discriminative purposes is obtained by noting the -position of the yellow portion of the spectral image given in white -light. - -In the case of a stone such as that depicted in Fig. 22 images are -given by other pairs of facets, for instance _ta_ and _tc_, unless the -angle included by the former is too large. There might therefore be -some doubt, to which pair some particular image corresponded; but no -confusion can arise if the following procedure be adopted. - -[Illustration: FIG. 23.—Course of Observations in the Method of Minimum -Deviation.] - -The table, or some easily recognizable facet, is selected as the facet -at which light enters the stone. The telescope is first placed in the -position in which it is directly opposite the collimator (_T_{0}_ in -Fig. 23), and clamped. The scale is turned until it reads exactly -zero, 0° or 360°, and clamped. The telescope is released and revolved -in the direction of increasing readings of the scale to the position -of minimum deviation, _T_. The reading of the scale gives at once the -angle of minimum deviation, _D_. The holder carrying the stone is now -clamped to the scale, and the telescope is turned to the position, -_T_{1}_,in which the image given by reflection from the table facet is -in the centre of the field of view; the reading of the scale is taken. -The telescope is clamped, and the scale is released and rotated until -it reads the angle already found for _D_. If no mistake has been made, -the reflected image from the second facet is now in the field of view. -It will probably not be quite central, as theoretically it should be, -because the stone may not have been originally quite in the position -of minimum deviation, a comparatively large rotation of the stone -producing no apparent change in the position of the refracted image at -minimum deviation, and further, because, as has already been stated, -the method is not strictly true for biaxial stones. The difference in -readings, however, should not exceed 2°. The reading, _S_, of the scale -is now taken, and it together with 180° subtracted from the reading for -the first facet, and the value of _A_, the interior angle between the -two facets, obtained. - -Let us take an example. - - Reading _T_ (= _D_) 40° 41´ Reading T_{1} 261° 35´ - less 180° 180 0 - ——————— - 81 35 - Reading _S_ 41 30 - ——————— - ½_D_ 20 20½ _A_ 40 5 - ½_A_ 20 2½ ½_A_ 20 2½ - ——————— - ½(_A_ + _D_) 40 23 - - Log sin 40° 23´ 9.81151 - Log sin 20 2½ 9.53492 - ——————— - Log _n_ 0.27659 - - _n_ = 1.8906. - -The readings _S_ and _T_ are very nearly the same, and therefore we may -be sure that no mistake has been made in the selection of the facets. - -In place of logarithm-tables we may make use of the diagram on Plate -II. The radial lines correspond to the angles of minimum deviation and -the skew lines to the prism angles, and the distance along the radial -lines gives the refractive index. We run our eye along the line for the -observed angle of minimum deviation and note where it meets the curve -for the observed prism angle; the refractive index corresponding to the -point of intersection is at once read off. - -This method has several obvious disadvantages: it requires the use -of an expensive and elaborate instrument, an observation takes -considerable time, and the values of the principal refractive indices -cannot in general be immediately determined. - -Table III at the end of the book gives the refractive indices of the -gem-stones. - -[Illustration: _PLATE II_ - -REFRACTIVE INDEX DIAGRAM] - - - - - CHAPTER V - - LUSTRE AND SHEEN - - -It has been already stated that whenever light in one medium falls -upon the surface separating it from another medium some of the light -is reflected within the first, while the remainder passes out into -the second medium, except when the first is of lower refractivity -than the second and light falls at an angle greater than that of -total-reflection. Similarly, when light impinges upon a cut stone -some of it is reflected and the remainder passes into the stone. What -is the relative amount of reflected light depends upon the nature of -the stone—its refractivity and hardness—and determines its lustre; -the greater the amount the more lustrous will the stone appear. There -are different kinds of lustre, and the intensity of each depends on -the polish of the surface. From a dull, _i.e._ an uneven, surface the -reflected light is scattered, and there are no brilliant reflections. -All gem-stones take a good polish, and have therefore, so long as the -surface retains its polish, considerable brilliancy; turquoise, on -account of its softness, is always comparatively dull. - -The different kinds of lustre are— - - (1) Adamantine, characteristic of diamond. - (2) Vitreous, as seen on the surface of fractured glass. - (3) Resinous, as shown by resins. - -Zircon and demantoid, the green garnet called by jewellers “olivine,” -alone among gem-stones have a lustre approaching that of diamond. The -remainder all have a vitreous lustre, though varying in degree, the -harder and the more refractive species being on the whole the more -lustrous. - -Some stones—for instance, a cinnamon garnet—appear to have a certain -greasiness in the lustre, which is caused by stray reflections from -inclusions or other breaks in the homogeneity of the interior. A pearly -lustre, which arises from cleavage cracks and is typically displayed -by the cleavage face of topaz, would be seen in a cut stone only when -flawed. - -Certain corundums when viewed in the direction of the -crystallographical axis display six narrow lines of light radiating at -angles of 60° from a centre in a manner suggestive of the conventional -representations of stars. Such stones are consequently known as -asterias, or more usually star-stones—star-rubies or star-sapphires, as -the case may be, and the phenomenon is called asterism. These stones -have not a homogeneous structure, but contain tube-like cavities -regularly arranged at angles of 60° in planes at right angles to the -crystallographical axis. The effect is best produced when the stones -are cut _en cabochon_ perpendicular to that axis. - -Chatoyancy is a somewhat similar phenomenon, but in this case the -fibres or cavities are parallel to a single direction, and a single -broadish band is displayed at right angles to it. Cat’s-eyes, as these -stones are termed, are cut _en cabochon_ parallel to the fibres. The -true cat’s-eye (Plate XXIX, Fig. 1) is a variety of chrysoberyl, but -the term is also often applied to quartz showing a similar appearance. -The latter is really a fibrous mineral, such as asbestos, which has -become converted into silica. The beautiful tiger’s-eye from South -Africa is a silicified crocidolite, the original blue colour of which -has been altered by oxidation to golden brown. Recently tourmalines -have been discovered which are sufficiently fibrous in structure to -display an effective chatoyancy. - -The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to -reflections from twin lamellæ. The wonderful iridescence which is -the glory of opal, and is therefore termed opalescence, arises from -a structure which is peculiar to that species. Opal is a solidified -jelly; on cooling it has become riddled with extremely thin cracks, -which were subsequently filled with similar material of slightly -different refractivity, and thus it consists of a series of films. At -the surface of each film interference of light takes place just as -at the surface of a soap-bubble, and the more evenly the films are -spaced apart the more uniform is the colour displayed, the actual tint -depending upon the thickness of the films traversed by the light giving -rise to the phenomenon. - - - - - CHAPTER VI - - DOUBLE REFRACTION - - -The optical phenomenon presented by many gem-stones is complicated -by their property of splitting up a beam of light into two with, in -general, differing characters. In this chapter we shall discuss the -nature of double refraction, as it is termed, and methods for its -detection. The phenomenon is not one that comes within the purview of -everyday experience. - -So long ago as 1669 a Danish physician, by name Bartholinus, noticed -that a plate of the transparent mineral which at that time had -recently been brought over from Iceland, and was therefore called -“Iceland-spar,” possessed the remarkable property of giving a double -image of objects close to it when viewed through it. Subsequent -investigation has shown that much crystallized matter is doubly -refractive, but in calcite—to use the scientific name for the species -which includes Iceland-spar—alone among common minerals is the -phenomenon so conspicuous as to be obvious to the unaided eye. The -apparent separation of the pair of images given by a plate cut or -cleaved in any direction depends upon its thickness. The large mass, -upwards of two feet (60 cm.) in thickness, which is exhibited at -the far end of the Mineral Gallery of the British Museum (Natural -History), displays the separation to a degree that is probably unique. - -[Illustration: FIG. 24.—Apparent doubling of the Edges of a Peridot -when viewed through the Table-Facet.] - -Although none of the gem-stones can emulate calcite in this character, -yet the double refraction of certain of them is large enough to be -detected without much difficulty. In the case of faceted stones the -opposite edges should be viewed through the table-facet, and any signs -of doubling noted. The double refraction of sphene is so large, viz. -0·08, that the doubling of the edges is evident to the unaided eye. -In peridot (Fig. 24), zircon (b), and epidote the apparent separation -of the edges is easily discerned with the assistance of an ordinary -lens. A keen eye can detect the phenomenon even in the case of such -substances as quartz with small double refraction. It must, however, be -remembered that in all such stones the refraction is single in certain -directions, and the amount of double refraction varies therefore -with the direction from nil to the maximum possessed by the stone. -Experiment with a plate of Iceland-spar shows that the rays transmitted -by it have properties differing from those of ordinary light. On -superposing a second plate we notice that there are now two pairs of -images, which are in general no longer of equal brightness, as was the -case before. If the second plate be rotated with respect to the first, -two images, one of each pair, disappear, and then the other two, the -plate having turned through a right angle between the two positions of -extinction; midway between these positions the images are all equally -bright. This variation of intensity implies that each of the rays -emerging from the first plate has acquired a one-sided character, or, -as it is usually expressed, has become plane-polarized, or, shortly, -polarized. - -[Illustration: FIG. 25.—Wave-Motion.] - -Before the discovery of the phenomenon of double refraction the -foundation of the modern theory of light had been laid by the genius of -Huygens. According to this theory light is the result of a wave-motion -(Fig. 25) in the ether, a medium that pervades the whole of space -whether occupied by matter or not, and transmits the wave-motion at a -rate varying with the matter with which it happens to coincide. Such -a medium has been assumed because it explains satisfactorily all the -phenomena of light, but it by no means follows that it has a concrete -existence. Indeed, if it has, it is so tenuous as to be imperceptible -to the most delicate experiments. The wave-motion is similar to that -observed on the surface of still water when disturbed by a stone flung -into it. The waves spread out from the source of disturbance; but, -although the waves seem to advance, the actual particles of water -merely move up and down, and have no motion at all in the direction -in which the waves are moving. If we imagine similar motion to take -place in any plane and not only the horizontal, we form some idea -of the nature of ordinary light. But after passing through a plate -of Iceland-spar, light no longer vibrates in all directions, but in -each beam the vibrations are parallel to a particular plane, the two -planes being at right angles. The exact relation of the direction of -the vibrations to the plane of polarization is uncertain, although it -undoubtedly lies in the plane containing the direction of the ray of -light and the perpendicular to the plane of polarization. The waves for -different colours differ in their length, _i.e._ in the distance, 2 -_bb_ (Fig. 25), from crest to crest, while the velocity, which remains -the same for the same medium, is proportional to the wave-length. The -intensity of the light varies as the square of the amplitude of the -wave, _i.e._ the height, _ab_, of the crest from the mean level. - -Various methods have been proposed for obtaining polarized light. Thus -Seebeck found in 1813 that a plate of brown tourmaline cut parallel -to the crystallographic axis and of sufficient thickness (cf. p. 11) -transmits only one ray, the other being entirely absorbed within the -plate. Another method was to employ a glass plate to reflect light at a -certain critical angle. The most efficient method, and that in general -use at the present day, is due to the invention of Nicol. A rhomb of -Iceland-spar (Fig. 26), of suitable length, is sliced along the longer -diagonal, _dd_, and the halves are cemented together by means of canada -balsam. One ray, _ioo_, is totally reflected at the surface separating -the mineral and the cement, and does not penetrate into the other half; -while the other ray, _iee_, is transmitted with almost undiminished -intensity. Such a rhomb is called a Nicol’s prism after its inventor, -or briefly, a nicol. - -[Illustration: FIG. 26.—Nicol’s Prism.] - -If one nicol be placed above another and their corresponding principal -planes be at right angles no light is transmitted through the pair. -In the polarizing microscope one such nicol, called the polarizer, is -placed below the stage, and the other, called the analyser, is either -inserted in the body of the microscope or placed above the eyepiece, -and the pair are usually set in the crossed position so that the field -of the microscope is dark. If a piece of glass or a fragment of some -singly refractive substance be placed on the stage the field still -remains dark; but in case of a doubly refractive stone the field is -no longer dark except in certain positions of the stone. On rotation -of the plate, or, if possible, of the nicols together, the field -passes from darkness to maximum brightness four times in a complete -revolution, the relative angular intervals between these positions -being right angles. These positions of darkness are known as the -positions of extinction, and the plate is said to extinguish in them. -This test is exceedingly delicate and reveals the double refraction -even when the greatest difference in the refractive indices is too -small to be measured directly. - -Doubly refractive substances are of two kinds: uniaxial, in which -there is one direction of single refraction, and biaxial, in which -there are two such directions. In the case of the former the direction -of one, the ordinary ray, is precisely the same as if the refraction -were single, but the refractive index of the other ray varies from -that of the ordinary ray to a second limiting value, the extraordinary -refractive index, which may be either greater or less. If the -extraordinary is greater than the ordinary refractive index the double -refraction is said to be positive; if less, to be negative. A biaxial -substance is more complex. It possesses three principal directions, -viz., the bisectrices of the directions of single refraction and the -perpendicular to the plane containing them. The first two correspond -to the greatest and least, and the last to the mean of the principal -indices of refraction. If the acute bisectrix corresponds to the -least refractive index, the double refraction is said to be positive, -and if to the greatest, negative. The relation of the directions of -single refraction, _s_, to the three principal directions, _a, b, c_, -is illustrated in Fig. 27 for the case of topaz, a positive mineral. -The refractive indices of the rays traversing one of the principal -directions have the values corresponding to the other two. In the -direction _a_ we should measure the greatest and the mean of the -principal refractive indices, in the direction _b_ the greatest and the -least, and in the direction _c_ the mean and the least. The maximum -amount of double refraction is therefore in the direction _b_. - -[Illustration: FIG. 27.—Relation of the two Directions of single -Refraction to the principal Optical Directions in a Biaxial Crystal.] - -In the examination of a faceted stone, of the most usual shape, the -simplest method is to lay the large facet, called the table, on a glass -slip and view the stone through the small parallel facet, the culet. -Should the latter not exist, it may frequently happen that owing to -internal reflection no light emerges through the steeply inclined -facets. This difficulty is easily overcome by immersing the stone in -some highly refracting oil. A glass plate held by hand over the stone -with a drop of the oil between it and the plate serves the purpose, and -is perhaps a more convenient method. A stone which does not possess a -pair of parallel facets should be viewed through any pair which are -nearly parallel. - -We have stated that a plate of glass has no effect on the field. -Suppose, however, it were viewed when placed between the jaws of a -tightened vice and thus thrown into a state of strain, it would then -show double refraction, the amount of which would depend on the strain. -Natural singly refractive substances frequently show phenomena of a -similar kind. Thus diamond sometimes contains a drop of liquid carbonic -acid, and the strain is revealed by the coloured rings surrounding -the cavity which are seen when the stone is viewed between crossed -nicols. Double refraction is also common in diamond even when there is -no included matter to explain it, and is caused by the state of strain -into which the mineral is thrown on release from the enormous pressure -under which it was formed. Other minerals which display these so-called -optical anomalies, such as fluor and garnet, are not really quite -singly refractive at ordinary temperatures; each crystal is composed -of several double refractive individuals. But all such phenomena -cannot be confused with the characters of minerals which extinguish -in the ordinary way, since the stone will extinguish in small patches -and these will not be dark all at the same time; further, the double -refraction is small, and on revolving the stone between crossed nicols -the extinction is not sharp. Paste stones are sometimes in a state of -strain, and display slight, but general, double refraction. Hence the -existence of double refraction does not necessarily prove that the -stone is real and not an imitation. Stones may be composed of two or -more individuals which are related to each other by twinning, in which -case each individual would in general extinguish separately. Such -individuals would be larger and would extinguish more sharply than the -patches of an anomalous stone. - -[Illustration: FIG. 28.—Interference of Light.] - -An examination in convergent light is sometimes of service. An -auxiliary lens is placed over the condenser so as to converge the light -on to the stone. Light now traverses the stone in different directions; -the more oblique the direction the greater the distance traversed in -the stone. If it be doubly refractive, in any given direction there -will be in general two rays with differing refractive indices and the -resulting effect is akin to the well-known phenomenon of Newton’s -rings, and is an instance of what is termed interference. It may be -mentioned that the interference of light (Fig. 28) explains such common -phenomena as the colours of a soap-bubble, the hues of tarnished steel, -the tints of a layer of oil floating on water, and so on. Light, after -diverging from the stone, comes to focus a little beneath the plane -in which the image of the stone is formed. An auxiliary lens must, -therefore, be inserted to bring the focal planes together, so that the -interference picture may be viewed by means of the same eyepiece. - -If a uniaxial crystal be examined along the crystallographic axis in -convergent light an interference picture will be seen of the kind -illustrated on Plate III. The arms of a black cross meet in the centre -of the field, which is surrounded by a series of circular rings, -coloured in white light. Rotation of the stone about the axis -produces no change in the picture. - -[Illustration: _PLATE III_ - -1. UNIAXIAL - -2. UNIAXIAL (_Circular Polarization_) - -3. BIAXIAL (_Crossed Brushes_) - -4. BIAXIAL (_Hyperbolic Brushes_) - -INTERFERENCE FIGURES] - -A biaxial substance possesses two directions (_the optic axes_) along -which a single beam is transmitted. If such a stone be examined along -the line bisecting the acute angle between the optic axes (_the acute -bisectrix_) an interference picture[4] will be seen which in particular -positions of the stone with respect to the crossed nicols takes the -forms illustrated on Plate III. As before, there is a series of rings -which are coloured in white light; they, however, are no longer circles -but consist of curves known as lemniscates, of which the figure of 8 -is a special form. Instead of an unchangeable cross there are a pair -of black “brushes” which in one position of the stone are hyperbolæ, -and in that at right angles become a cross. On rotating the stone we -find that the rings move with it and are unaltered in form, whereas the -brushes revolve about two points, called the “eyes,” where the optic -axes emerge. If the observation were made along the obtuse bisectrix -the angle between the optic axes would probably be too large for the -brushes to come into the field, and the rings might not be visible in -white light, though they would appear in monochromatic light. In the -case of a substance like sphene the figure is not so simple, because -the positions of the optic axes vary greatly for the different colours -and the result is exceedingly complex; in monochromatic light, however, -the usual figure is visible. - -It would probably not be possible in the case of a faceted stone -to find a pair of faces perpendicular to the required direction. -Nevertheless, so long as a portion of the figures described is in the -field of view, the character of the double refraction, whether uniaxial -or biaxial, may readily be determined. - -There is yet another remarkable phenomenon which must not be passed -over. Certain substances, of which quartz is a conspicuous example and -in this respect unique among the gem-stones, possess the remarkable -property of rotating the plane of polarization of a ray of light which -is transmitted parallel to the optic axis. If a plate of quartz be -cut at right angles to the axis and placed between crossed nicols in -white light, the field will be coloured, the hue changing on rotation -of one nicol with respect to the other. Examination in monochromatic -light shows that the field will become dark after a certain rotation of -the one nicol with respect to the other, the amount of which depends -on the thickness of the plate. If the plate be viewed in convergent -light, an interference picture is seen as illustrated on Plate III, -which is similar to, and yet differs in some important particulars -from the ordinary interference picture of a uniaxial stone. The cross -does not penetrate beyond the innermost ring and the centre of the -field is coloured in white light. If a stone shows such a picture, it -may be safely assumed to be quartz. It is interesting to note that -minerals which possess this property have a spiral arrangement of the -constituent atoms. - -It has already been remarked (p. 28) that if a faceted doubly -refractive stone be rotated with one facet always in contact with the -dense glass of the refractometer the pair of shadow-edges that are -visible in the field move up or down the scale in general from or -to maximum and minimum positions. The manner in which this movement -takes place depends upon the character of the double refraction and -the position of the facet under observation with regard to the optical -symmetry of the stone. In the case of a uniaxial stone, if the facet -be perpendicular to the crystallographic axis, i.e. the direction -of single refraction, neither of the shadow-edges will move. If the -facet be parallel to that direction, one shadow-edge will move up and -coincide with the other, which remains invariable in position, and -away from it to a second critical position; the latter gives the value -of the extraordinary refractive index, and the invariable shadow-edge -corresponds to the ordinary refractive index. This phenomenon is -displayed by the table-facet of most tourmalines, because for -reasons given above (p. 11) they are as a rule cut parallel to the -crystallographic axis. In the case of facets in intermediate positions, -the shadow-edge corresponding to the extraordinary refractive index -moves, but not to coincidence with the invariable shadow-edge. The case -of a biaxial stone is more complex. If the facet be perpendicular to -one of the principal directions one shadow-edge remains invariable in -position, corresponding to one of the principal refractive indices, -whilst the other moves between the critical values corresponding -to the remaining two of the principal refractive indices. In the -interesting case in which the facet is parallel to the two directions -of single refraction, the second shadow-edge moves across the one which -is invariable in position. In intermediate positions of the facet -both shadow-edges move, and give therefore critical values. Of the -intermediate pair, _i.e._ the lower maximum and the higher minimum, -one corresponds to the mean principal refractive index, and the other -depends upon the relation of the facet to the optical symmetry. If it -is desired to distinguish between them, observations must be made on -a second facet; but for discriminative purposes such exactitude is -unnecessary, since the least and the greatest refractive indices are -all that are required. - -The character of the refraction of gem-stones is given in Table V at -the end of the book. - - - - - CHAPTER VII - - ABSORPTION EFFECTS: COLOUR, DICHROISM, ETC. - - -When white light passes through a cut stone, colour effects result -which arise from a variety of causes. The most obvious is the -fundamental colour of the stone, which is due to its selective -absorption of the light passing through it, and would characterize -it before it was cut. Intermingled with the colour in a transparent -stone is the dispersive effect known as ‘fire,’ which has already -been discussed (p. 20). In many instances the want of homogeneity is -responsible for some peculiar effects such as opalescence, chatoyancy, -and asterism. These phenomena will now be considered in fuller detail. - - - COLOUR - -All substances absorb light to some extent. If the action is slight and -affects equally the whole of the visible spectrum, the stone appears -white or colourless. Usually some portion is more strongly absorbed -than the rest, and the stone seems to be coloured. What is the precise -tint depends not only upon the portions transmitted through the stone, -but also upon their relative intensities. The eye, unlike the ear, has -not the power of analysis and it cannot of itself determine how a -composite colour has been made up. Indeed, so far as it is concerned, -any colour may be exactly matched by compounding in certain proportions -three simple primary colours—red, yellow, and violet. Alexandrite, a -variety of chrysoberyl, is a curious and instructive case. The balance -in the spectrum of light transmitted through it is such that, whereas -in daylight such stones appear green, in artificial light, especially -in gas-light, they are a pronounced raspberry-red (Plate XXVII, -Figs. 11, 13). The phenomenon is intensified by the strong dichroism -characteristic of this species. - -The colour is the least reliable character that may be employed for the -identification of a stone, since it varies considerably in the same -species, and often results from the admixture of some metallic oxide, -which has no essential part in the chemical composition and is present -in such minute quantities as to be almost imperceptible by analysis. -Who would, for instance, imagine from their appearance that stones so -markedly diverse in hue as ruby and sapphire were really varieties of -the same species, corundum? Again, quartz, in spite of the simplicity -of its composition, displays extreme differences of tint. Nevertheless, -certain varieties do possess a distinctive colour, emerald being -the most striking example, and in other cases the trained eye can -appreciate certain characteristic subtleties of shade. At any rate, the -colour is the most obvious of the physical characters, and serves to -provide a rough division of the species, and accordingly in Table II at -the end of the book the gem-stones are arranged by their usual tints. - - - DICHROISM - -The two rays into which a doubly refractive stone splits up a ray of -light are often differently absorbed by it, and in consequence appear -on emergence differently coloured; such stones are said to be dichroic. -The most striking instance is a deep-brown tourmaline, which, except -in very thin sections, is quite opaque to the ordinary ray. The light -transmitted by a plate cut parallel to the crystallographic axis is -therefore plane-polarized; before the invention by Nicol of the prism -of Iceland-spar known by his name this was the ordinary method of -obtaining light of this character (cf. p. 43). Again, in the case of -kunzite and cordierite the difference in colour is so marked as to be -obvious to the unaided eye; but where the contrast is less pronounced -we require the use of an instrument called a dichroscope, which enables -the twin colours to be seen side by side. - -[Illustration: FIG. 29.—Dichroscope (actual size).] - -[Illustration: FIG. 30.—Field of the Dichroscope.] - -Fig. 29 illustrates in section the construction of a dichroscope. The -instrument consists essentially of a rhomb of Iceland-spar, _S_, of -such a length as to give two contiguous images (Fig. 30) of a square -hole, _H_, in one end of the tube containing it. In some instruments -the terminal faces of the rhomb are ground at right angles to its -length, but usually, as in that depicted, prisms of glass, _G_, are -cemented on to the two ends. A cap _C_, with a slightly larger hole, -which is circular in shape, fits on the end of the tube, and can be -moved up and down it and revolved round it, as desired. The stone, _R_, -to be tested may be directly attached to it by means of some kind of -wax or cement in such a way that light which has traversed it passes -into the window, _H_, of the instrument; the cap at the same time -permits of the rotation of the stone about the axis of the main tube of -the instrument. The dichroscope shown in the figure has a still more -convenient arrangement: it is provided with an additional attachment, -_A_, by means of which the stone can be turned about an axis at right -angles to the length of the tube, and thus examined in different -directions. At the other end of the main tube is placed a lens, _L_, of -low power for viewing the twin images: the short tube containing it can -be pushed in and out for focusing purposes. Many makers now place the -rhomb close to the lens, _L_, and thereby require a much smaller piece -of spar; material suitable for optical purposes is fast growing scarce. - -Suppose that a plate of tourmaline cut parallel to its crystallographic -axis is fastened to the cap and the latter rotated. We should notice, -on looking through the instrument, that in the course of a complete -revolution there are two positions, orientated at right angles to -one another, in which the tints of the two images are identical, the -positions of greatest contrast of tint being midway between. If we -examine a uniaxial stone in a direction at right angles to its optic -axis we obtain the colours corresponding to the ordinary and the -extraordinary rays. In any direction less inclined to the axis we -still have the colour for the ordinary ray, but the other colour is -intermediate in tint between it and that for the extraordinary ray. -The phenomenon presented by a biaxial stone is more complex. There -are three principal colours which are visible in differing pairs in -the three principal optical directions; in other directions the tints -seen are intermediate between the principal colours. Since biaxial -stones have three principal colours, they are sometimes said to be -trichroic or pleochroic, but in any single direction they have two -twin colours and show dichroism. No difference at all will be shown in -directions in which a stone is singly refractive, and it is therefore -always advisable to examine a stone in more than one direction lest -the first happens to be one of single refraction. For determinative -purposes it is not necessary to note the exact shades of tint of the -twin colours, because they vary with the inherent colour of the stone, -and are therefore not constant for the same species; we need only -observe, when the stone is tested with the dichroscope, whether there -is any variation of colour, and, if so, its strength. Dichroism is a -result of double refraction, and cannot exist in a singly refractive -stone. The converse, however, is not true and it by no means follows -that, because no dichroism can be detected in a stone, it is singly -refractive. A colourless stone, for instance, cannot possibly be -dichroic, and many coloured, doubly refractive stones—for example, -zircon—exhibit no dichroism, or so little that it is imperceptible. -The character is always the better displayed, the deeper the inherent -colour of the stone. The deep-green alexandrite, for instance, is far -more dichroic than the lighter coloured varieties of chrysoberyl. - -If the stone is attached to the cap of the instrument, the table should -be turned towards it so as to assure that the light passing into the -instrument has actually traversed the stone. If little light enters -through the opposite coign, a drop of oil placed thereon will overcome -the difficulty (cf. p. 46). It is also necessary, for reasons mentioned -above, to examine the stone in directions as far as possible across -the girdle also. A convenient, though not strictly accurate, method -is to lay the stone with the table facet on a table and examine the -light which has entered the stone and been reflected at that facet. The -stone may easily be rotated on the table, and observations thus made -in different directions in the stone. Care must be exercised in the -case of a faceted stone not to mistake the alteration in colour due to -dispersion for a dichroic effect, and the stone must be placed close to -the instrument during an observation, because otherwise the twin rays -traversing the instrument may have taken sensibly different directions -in the stone. - -Dichroism is an effective test in the case of ruby; its twin -colours—purplish and yellowish red—are in marked contrast, and readily -distinguish it from other red stones. Again, one of the twin colours -of sapphire is distinctly more yellowish than the other; the blue -spinel, of which a good many have been manufactured during recent -years, is singly refractive, and, of course, shows no difference of -tint in the dichroscope. - -Table VI at the end of the book gives the strength of the dichroism of -the gem-stones. - - - ABSORPTION SPECTRA - -A study of the chromatic character of the light transmitted by a -coloured stone is of no little interest. As was stated above, the eye -has not the power of analysing light, and to resolve the transmitted -rays into their component parts an instrument known as a spectroscope -is needed. The small ‘direct-vision’ type has ample dispersion for this -purpose. It is advantageous to employ by preference the diffraction -rather than the prism form, because in the former the intervals in the -resulting spectrum corresponding to equal differences of wave-length -are the same, whereas in the latter they diminish as the wave-length -increases and accordingly the red end of the spectrum is relatively -cramped. - -The absorptive properties of all doubly refractive coloured substances -vary more or less with the direction in which light traverses them -according to the amount of dichroism that they possess, but the -variation is not very noticeable unless the stone is highly dichroic. -If the light transmitted by a deep-coloured ruby be examined with a -spectroscope it will be found that the whole of the green portion of -the spectrum is obliterated (Fig. 31), while in the case of a sapphire -only a small portion of the red end of the spectrum is absorbed. -Alexandrite affords especial interest. In the spectrum of the light -transmitted by it, the violet and the yellow are more or less strongly -absorbed, depending upon the direction in which the rays have passed -through the stone (Fig. 31), and the transmitted light is mainly -composed of two portions—red and green. The apparent colour of the -stone depends, therefore, upon which of the two predominates. In -daylight the resultant colour is green flecked with red and orange, -the three principal absorptive tints (cf. p. 235), but in artificial -light, which is relatively stronger in the red portion of the spectrum, -the resultant colour is a raspberry-red, and there is less apparent -difference in the absorptive tints (cf. Plate XXVII, Figs. 11, 13). - -[Illustration: FIG. 31.—Absorption Spectra.] - -In all the spectra just considered, and in all like them, the portions -that are absorbed are wide, the passage from blackness to colour is -gradual, and the edges deliminating them are blurred. In the spectra of -certain zircons and in almandine garnet the absorbed portions, or bands -as they are called, are narrow, and, moreover, the transition from -blackness to colour is sharp and abrupt; such stones are therefore said -to display absorption-bands. Church in 1866 was the first to notice -the bands shown by zircon (Fig. 31). Sorby thought they portended the -existence of a new element, to which he gave the name jargonium, but -subsequently discovered that they were caused by the presence of a -minute trace of uranium. A yellowish-green zircon shows the phenomenon -best, and it has all the bands shown in the figure. The spectrum varies -slightly but almost imperceptibly with the direction in the stone. -Others show the bands in the yellow and green, while others show only -those in the red, and some only one of them. The bands are not confined -to stones of any particular colour, or amount of double refraction. -Again, many zircons show no bands at all, so that their absence by no -means precludes the stone from being a zircon. - -Almandine is characterized by a different spectrum (Fig. 31). The band -in the yellow is the most conspicuous, and is no doubt responsible for -the purple hue of a typical almandine. The spectrum varies in strength -in different stones. Rhodolite (p. 214), a garnet lying between -almandine and pyrope, displays the same bands, and indications of them -may be detected in the spectra of pyropes of high refraction. - -[Illustration: _PLATE IV_ - -JEWELLERY DESIGNS] - - - - - CHAPTER VIII - - SPECIFIC GRAVITY - - -It is one of our earliest experiences that different substances of -the same size have often markedly different weights; thus, there is -a great difference between wood and iron, and still greater between -wood and lead. It is usual to say that iron is heavier than wood, -but the statement is misleading, because it would be possible by -selecting a large enough piece of wood to find one at least as heavy -as a particular piece of iron. We have, in fact, to compare equal -volumes of the two substances, and all ambiguity is removed if we -speak of relative density or specific gravity—the former term being -usually applied to liquids and the latter to solids—instead of weight -or heaviness. The density of water at 4° C. is taken as unity, that -being the temperature at which it is highest; at other temperatures it -is somewhat lower, as will be seen from Table IX given at the end of -the book. The direct determination of the volume of an irregular solid -presents almost insuperable difficulty; but, fortunately, for finding -the specific gravity it is quite unnecessary to know the volume, as -will be shown when we proceed to consider the methods in use. - -The specific gravity of a stone is a character which is within narrow -limits constant for each species, and is therefore very useful for -discriminative purposes. It can be determined whatever be the shape -of the stone, and it is immaterial whether it be transparent or not; -but, on the other hand, the stone must be unmounted and free from the -setting. - -The methods for the determination of the specific gravity are of two -kinds: in the first a liquid is found of the same, or nearly the same, -density as the stone, and in the second weighings are made and the use -of an accurate balance is required. - - - (1) HEAVY LIQUIDS - -Experiment tells us that a solid substance floats in a liquid denser -than itself, sinks in one less dense, and remains suspended at any -level in one of precisely the same density. If the stone be only -slightly less dense than the liquid, it will rise to the surface; if -it be just as slightly denser, it will as surely sink to the bottom, -a physical fact which has added so much to the difficulty and danger -of submarine manœuvring. If then we can find a liquid denser than the -stone to be tested, and place the latter in it, the stone will float on -the surface. If we take a liquid which is less dense than the stone and -capable of mixing with the heavier liquid, and add it to the latter, -drop by drop, gently stirring so as to assure that the density of -the combination is uniformly the same throughout, a stage is finally -reached when the stone begins to move downwards. It has now very nearly -the density of the liquid, and, if we find by some means this density, -we know simultaneously the specific gravity of the stone. - -Various devices and methods are available for ascertaining the density -of liquids—for instance, Westphal’s balance; but, apart from the -inconvenience attending such a determination, the density of all -liquids is somewhat seriously affected by changes in the temperature, -and it is therefore better to make direct comparison with fragments -of substances of known specific gravity, which are termed indicators. -If of two fragments differing slightly in specific gravity one floats -on the surface of a uniform column of liquid and the other lies at -the bottom of the tube containing the liquid, we may be certain that -the density of the liquid is intermediate between the two specific -gravities. Such a precaution is necessary because, if the liquid be a -mixture of two distinct liquids, the density would tend to increase -owing to the greater volatility of the lighter of them, and in any case -the density is affected by change of temperature. The specific gravity -of stones is not much altered by variation in the temperature. - -A more convenient variation of this method is to form a diffusion -column, so that the density increases progressively with the depth. -If the stone under test floats at a certain level in such a column -intermediate between two fragments of known specific gravity, its -specific gravity may be found by elementary interpolation. To form a -column of this kind the lighter liquid should be poured on to the top -of the heavier. Natural diffusion gives the most perfect column, but, -being a lengthy process, it may conveniently be quickened by gently -shaking the tube, and the column thus formed gives results sufficiently -accurate for discriminative purposes. - -By far the most convenient liquid for ordinary use is methylene -iodide, which has already been recommended for its high refraction. -It has, when pure, a density at ordinary room-temperatures of 3·324, -and it is miscible in all proportions with benzol, whose density is -O·88, or toluol, another hydrocarbon which is somewhat less volatile -than benzol, and whose density is about the same, namely, 0·86. When -fresh, methylene iodide has only a slight tinge of yellow, but it -rapidly darkens on exposure to light owing to the liberation of iodine -which is in a colloidal form and cannot be removed by filtration. -The liquid may, however, be easily cleared by shaking it up with any -substance with which the iodine combines to form an iodide removable -by filtration. Copper filings answer the purpose well, though rather -slow in action; mercury may also be used, but is not very satisfactory, -because a small amount may be dissolved and afterwards be precipitated -on to the stone under test, carrying it down to the bottom of the tube. -Caustic potash (potassium hydroxide) is also recommended; in this case -the operation should preferably be carried out in a special apparatus -which permits the clear liquid to be drawn off underneath, because -water separates out and floats on the surface. In Fig. 32 three cut -stones, a quartz (_a_), a beryl (_b_), and a tourmaline (_c_) are shown -floating in a diffusion column of methylene iodide and benzol. Although -the beryl is only slightly denser than the quartz, it floats at a -perceptibly lower level. These three species are occasionally found as -yellow stones of very similar tint. - -[Illustration: FIG. 32.—Stones of different Specific Gravities floating -in a Diffusion Column of heavy Liquid.] - -Various other liquids have been used or proposed for the same -purpose, of which two may be mentioned. The first of them is a -saturated solution of potassium iodide and mercuric iodide in water, -which is known after the discoverer as Sonstadt’s solution. It is a -clear mobile liquid with an amber colour, having at 12° C. a density -of 3·085; it may be mixed with water to any extent, and is easily -concentrated by heating; moreover, it is durable and not subject to -alteration of any kind; on the other hand, it is highly poisonous and -cauterizes the skin, not being checked by albumen; it also destroys -brass-ware by amalgamating the metal. The second is Klein’s solution, -a clear yellow liquid which has at 15° C. a density of 3·28. It -consists of the boro-tungstate of cadmium, of which the formula is -9WO_{3}.B_{2}O_{3}.2CdO.2H_{2}O + 16Aq, dissolved in water, with which -it may be diluted. If the salt be heated, it fuses at 75° C. in its -own water of crystallization to a yellow liquid, very mobile, with a -density of 3·55. Klein’s solution is harmless, but it cannot compare -for convenience of manipulation with methylene iodide. - -The most convenient procedure is to have at hand three glass tubes, -fitted with stoppers or corks, to contain liquids of different -densities— - -(_a_) Methylene iodide reduced to 2·7; using as indicators orthoclase -2·55, quartz 2·66, and beryl 2·74. - -(_b_) Methylene iodide reduced to 3·1; indicators, beryl 2·74 and -tourmaline 3·10. - -(_c_) Methylene iodide, undiluted, 3·32. - -The pure liquid in the last tube should on no account be diluted; but -the density of the other two liquids may be varied slightly, either -by adding benzol in order to lower it, or by allowing benzol, which -has far greater volatility than methylene iodide, to evaporate, or by -adding methylene iodide, in order to increase it. The density of the -liquids may be ascertained approximately from the indicators. - -A glance at the table of specific gravities shows that as regards the -gem-stones methylene iodide is restricted in its application, since -it can be used to test only moonstone, quartz, beryl, tourmaline, and -spodumene; opal and turquoise, being amorphous and more or less porous, -should not be immersed in liquids, lest the appearance of the stone be -irretrievably injured. Methylene iodide readily serves to distinguish -the yellow quartz from the true topaz, with which jewellers often -confuse it, the latter stone sinking in the liquid; again aquamarine -floats, but the blue topaz, which is often very similar to it, sinks in -methylene iodide. - -By saturating methylene iodide with iodine and iodoform, we have a -liquid (_d_) of density 3·6; a fragment of topaz, 3·55, may be used to -indicate whether the liquid has the requisite density. Unfortunately -this saturated solution is so dark as to be almost opaque, and is, -moreover, very viscous. Its principal use is to distinguish diamond, -3·535, from the brilliant colourless zircon, with which, apart from -a test for hardness, it may easily be confused. It is easy to see -whether the stone floats, as it would do if a diamond. To recover a -stone which has sunk, the only course is to pour off the liquid into -another tube, because it is far too dark for the position of the stone -to be seen. - -It is possible to employ a similar method for still denser stones by -having recourse to Retgers’s salt, silver-thallium nitrate. This double -salt is solid at ordinary room-temperatures, but has the remarkable -property of melting at a temperature, 75° C., which is well below the -point of fusion of either of its constituents, to a clear, mobile -yellow liquid, which is miscible in any proportion with water, and -has, when pure, a density of 4·6. The salt may be purchased, or it may -be prepared by mixing 100 grams of thallium nitrate and 64 grams of -silver nitrate, or similar proportions, in a little water, and heating -the whole over a water-bath, keeping it constantly stirred with a -glass rod until it is liquefied. The two salts must be mixed in the -correct proportions, because otherwise the mixture might form other -double salts, which do not melt at so low a temperature. A glance at -the table of specific gravities shows that Retgers’s salt may be used -for all the gem-stones with the single exception of zircon (b). There -are, however, some objections to its use. It is expensive, and, unless -kept constantly melted, it is not immediately available. It darkens on -exposure to strong sunlight like all silver salts, stains the skin a -peculiar shade of purple which is with difficulty removed, and in fact -only by abrasion of the skin, and, like all thallium compounds, is -highly poisonous. - -It is convenient to have three tubes, fitted as before with stoppers -or corks, to contain the following liquids, when heated:— - -(_e_) Silver-thallium nitrate, reduced to 3·5; using as indicators, -peridot or idocrase 3·40 and topaz 3·53. - -(_f_) Silver-thallium nitrate, reduced to 4·0; indicators, topaz 3·53 -and sapphire 4·03. - -(_g_) Silver-thallium nitrate, undiluted, 4·6. - -The tubes must be heated in some form of water-bath; an ordinary glass -beaker serves the purpose satisfactorily. The pure salt should never be -diluted; but the density of the contents of tubes (_e_) and (_f_) may -be varied at will, water being added in order to lower the density, and -concentration by means of evaporation or addition of the nitrate being -employed in order to increase it. To avoid the discoloration of the -skin, rubber finger-stalls may be used, and the stones should not be -handled until after they have been washed in warm water. The staining -may be minimized if the hands be well washed in hot water before being -exposed to sunlight. It is advisable to warm the stone to be tested -in a tube containing water beforehand lest the sudden heating develop -cracks. A piece of platinum, or, failing that, copper wire is of -service for removing stones from the tubes; a glass rod, spoon-shaped -at one end, does equally well. It must be noted that although Retgers’s -salt is absolutely harmless to the ordinary gem-stones—with the -exception of opal and turquoise, which, as has already been stated, -being to some extent porous, should not be immersed in liquids—it -attacks certain substances, for instance, sulphides and cannot be -applied indiscriminately to minerals. - -The procedure described above is intended only as a suggestion; -the method may be varied to any extent at will, depending upon the -particular requirements. If such tests are made only occasionally, a -smaller number of tubes may be used. Thus one tube may be substituted -for the two marked _a_ and _b_, the liquid contained in it being -diluted as required, and a series of indicators may be kept apart in -small glass tubes. On the other hand, any one having constantly to test -stones might increase the number of tubes with advantage, and might -find it useful to have at hand fragments of all the principal species -in order to make direct comparison. - - - (2) DIRECT WEIGHING - -The balance which is necessary in both the methods described under this -head should be capable of giving results accurate to milligrams, _i.e._ -the thousandth part of a gram, and consistent with that restriction -the beam may be as short as possible so as to give rapid swings and -thus shorten the time taken in the observations. A good assay balance -answers the purpose admirably. Of course, it is never necessary to wait -till the balance has come to rest. The mean of the extreme readings of -the pointer attached to the beam will give the position in which it -would ultimately come to rest. Thus, if the pointer just touches the -eighth division on the right-hand side and the second on the other, -the mean position is the third division on the right-hand side (½(8 − -2) = 3). Instead of the ordinary form of chemical balance, Westphal’s -form or Joly’s spring-balance may be employed. Weighings are made more -quickly, but are not so accurate. - -In refined physical work the practice known as double-weighing is -employed to obviate any slight error there may be in the suspension -of the balance. A counterpoise which is heavier than anything to be -weighed is placed in one pan, and weighed. The counterpoise is retained -in its pan throughout the whole course of the weighings. Any substance -whose weight is to be found is placed in the other pan, and weights -added till the balance swings truly again. The difference between -the two sets of weights evidently gives the weight of the substance. -Balances, however, are so accurately constructed that for testing -purposes such refined precautions are not really necessary. - -It is immaterial in what notation the weighings are made, so long as -the same is used throughout, but the metric system of weights, which -is in universal use in scientific work, should preferably be employed. -Jewellers, however, use carat weights, and a subdivision to the base -2 instead of decimals, the fractions being ½, ¼, ⅛, 1/16, 1/32, 1/64. -If these weights be employed, it will be necessary to convert these -fractions into decimals, and write ½ = ·500, ¼ ·250, ⅛ = ·125, 1/16 = -·062, 1/32 = ·031, 1/64 = ·016. - - - (a) _Hydrostatic Weighing_ - -The principle of this method is very simple. The stone, the specific -gravity of which is required, is first weighed in air and then when -immersed in water. If _W_ and _W´_ be these weights respectively, then -_W_ − _W´_ is evidently the weight of the water displaced by the stone -and having therefore the same volume as it, and the specific gravity is - _W_ -therefore equal to ———————————. - _W_ − _W^r_ - -If the method of double-weighing had been adopted, the formula would -be slightly altered. Thus, suppose that _c_ corresponds to the -counterpoise, _w_ and _w´_ to the stone weighed in air and water -respectively; then we have _W_ = _c_ − _w_ and _W´_ = _c_ − _w´_, and - _c_ − _w_ -therefore the specific gravity is equal to ——————————. - _w´_ − _w_ - -[Illustration: FIG. 33.—Hydrostatic Balance.] - -Some precautions are necessary in practice to assure an accurate -result. A balance intended for specific gravity work is provided with -an auxiliary pan (Fig. 33), which hangs high enough up to permit of -the stone being suspended underneath. The weight of anything used for -the suspension must, of course, be determined and subtracted from the -weight found for the stone, both when in air and when in water. A piece -of fine silk is generally used for suspending the stone in water, -but it should be avoided, because the water tends to creep up it and -the error thus introduced affects the first place of decimals in the -case of a one-carat stone, the value being too high. A piece of brass -wire shaped into a cage is much to be preferred. If the same cage be -habitually used, its weight in air and when immersed in water to the -customary extent in such determinations should be found once for all. - -Care must also be taken to remove all air-bubbles which cling to the -stone or the cage; their presence would tend to make the value too low. -The surface tension of water which makes it cling to the wire prevents -the balance swinging freely, and renders it difficult to obtain a -weighing correct to a milligram when the wire dips into water. This -difficulty may be overcome by substituting a liquid such as toluol, -which has a much smaller surface tension. - -As has been stated above, the density of water at 4° C. is taken as -unity, and it is therefore necessary to multiply the values obtained -by the density of the liquid, whatever it be, at the temperature of -the observation. In Table IX, at the end of the book, are given the -densities of water and toluol at ordinary room-temperatures. It will be -noticed that a correct reading of the temperature is far more important -in the case of toluol. - - - _Example of a Hydrostatic Determination of Specific Gravity—_ - - Weight of stone in air = 1·471 gram - Weight of stone in water = 1·067 „ - 1·471 1·471 - Specific gravity = ————————————— = ————— - 1·471 − 1·067 0·404. - -Allowing for the density of water at the temperature of the room, which -was 16° C., the specific gravity is 3·637. Had no such allowance been -made, the result would have been four units too high in the third place -of decimals. For discriminative purposes, however, such refinement is -unnecessary. - - - (b) _Pycnometer, or Specific Gravity Bottle_ - -The specific gravity bottle is merely one with a fairly long neck on -which a horizontal mark has been scratched, and which is closed by -a ground glass stopper. The pycnometer is a refined variety of the -specific gravity bottle. It has two openings: the larger is intended -for the insertion of the stone and the water, and is closed by a -stopper through which a thermometer passes, while the other, which is -exceedingly narrow, is closed by a stopper fitting on the outside, and -is graduated to facilitate the determination of the height of the water -in it. - -The stone is weighed as in the previous method. The bottle is then -weighed, and filled with water up to the mark and weighed again. The -stone is now introduced into the bottle, and the surplus water removed -with blotting-paper or otherwise until it is at the same level as -before, and the bottle with its contents is weighed. Let _W_ be the -weight of the stone, _w_ the weight of the bottle, _W_´ the weight of -the bottle and the water contained in it, and _W_″ the weight of the -bottle when containing the stone and the water. Then _W_´ − _w_ is the -weight of the water filling the bottle up to the mark, and -_W_″ − _w_ − _W_ is the reduced weight of water after the stone has -been inserted; the difference, _W_ + _W_´ − _W_″, is the weight of the - _W_ -water displaced. The specific gravity is therefore ——————————————————. - _W_ + _W_´ − _W_″ -As in the previous method, this value must be multiplied by the density -of the liquid at the temperature of the experiment. If the method of -double-weighing be adopted, the formula will be slightly modified. - - • • • • • - -Of the above methods, that of heavy liquids, as it is usually termed, -is by far the quickest and the most convenient for stones of ordinary -size, the specific gravity of which is less than the density of pure -methylene iodide, namely, 3·324, and by its aid a value may be obtained -which is accurate to the second place of decimals, a result quite -sufficient for a discriminative test. The method is applicable no -matter how small the stone may be, and, indeed, for very small stones -it is the only trustworthy method; for large stones it is inconvenient, -not only because of the large quantity of liquid required, but also -on account of the difficulty in estimating with sufficient certainty -the position of the centre of gravity of the stone. A negative -determination may be of value, especially if attention be paid to the -rate at which the stone falls through the liquid; the denser the stone -the faster it will sink, but the rate depends also upon the shape of -the stone. Retgers’s salt is less convenient because of the delay -involved in warming it and of the almost inevitable staining of the -hands, but its use presents no difficulty whatever. - -Hydrostatic weighing is always available, unless the stone be very -small, but the necessary weighings occupy considerable time, and care -must be taken that no error creeps into the computation, simple though -it be. Even if everything is at hand, a determination is scarcely -possible under a quarter of an hour. - -The third method, which takes even longer, is intended primarily for -powdered substances, and is not recommended for cut stones, unless -there happen to be a number of tiny ones which are known to be exactly -of the same kind. - -The specific gravities of the gem-stones are given in Table VII at the -end of the book. - - - - - CHAPTER IX - - HARDNESS AND CLEAVABILITY - - -Every possessor of a diamond ring is aware that diamond easily -scratches window-glass. If other stones were tried, it would be -found that they also scratched glass, but not so readily, and, if -the experiment were extended, it would be found that topaz scratches -quartz, but is scratched by corundum, which in its turn yields to -the all-powerful diamond. There is therefore considerable variation -in the capacity of precious stones to resist abrasion, or, as it is -usually termed, in their hardness. To simplify the mode of expressing -this character the mineralogist Mohs about a century ago devised the -following arbitrary scale, which is still in general use. - - - MOHS’S SCALE OF HARDNESS - - 1. Talc - 2. Gypsum - 3. Calcite - 4. Fluor - 5. Apatite - 6. Orthoclase - 7. Quartz - 8. Topaz - 9. Corundum - 10. Diamond - -A finger-nail scratches gypsum and softer substances. Ordinary -window-glass is slightly softer than orthoclase, and a steel knife is -slightly harder; a hardened file approaches quartz in hardness, and -easily scratches glass. - -By saying that a stone has hardness 7 we merely mean that it will not -scratch quartz, and quartz will not scratch it. The numbers indicate -an order, and have no quantitative significance whatever. This is an -important point about which mistakes are often made. We must not, for -instance, suppose that diamond has twice the hardness of apatite. -As a matter of fact, the interval between diamond and corundum is -immensely greater than that between the latter and talc, the softest -of mineral substances. Intermediate degrees of hardness are expressed -by fractions. The number 8½ for chrysoberyl means that it scratches -topaz as easily as it itself is scratched by corundum. Pyrope garnet is -slightly harder than quartz, and its hardness is said therefore to be -7¼. - -Delicate tests show that the structure of all crystallized substances -is more or less grained, like that of wood, and the hardness for the -same stone varies in different directions. Kyanite is unique in this -respect, since its hardness ranges from 5 to 7; it can therefore -be scratched by a knife in some directions, but not in others. In -most substances, however, the range is so small as to be quite -imperceptible. Slight variation is also apparent in the hardness of -different specimens of the same species. The diamonds from Borneo and -New South Wales are so distinctly harder than those from South Africa -and other localities that, when first discovered, some difficulty was -experienced in cutting them. Again, lapidaries find that while Ceylon -sapphires are harder than rubies, Kashmir sapphires are softer. - -Hardness is a character of fundamental importance in a stone intended -for ornamental wear, since upon it depends the durability of the polish -and brilliancy. Ordinary dust is largely composed of grains of sand, -which is quartz in a minute form, and a gem-stone should therefore -be at least as hard as that. Paste imitations are little harder than -5, and consequently, as experience shows, their polish does not -survive a few weeks’ wear. Hardness is, however, of little use as a -discriminative test except for distinguishing between topaz or harder -stone and paste. Diamond is so much harder than other stones that it -will leave a cut in glass quite different from the scratch of even -corundum. Paste, being so soft, readily yields to the file, and is thus -easily distinguished from genuine stones. In applying the test to a -cut stone, it is best to remove it from its mount and try the effect -on the girdle, because any scratch would be concealed afterwards by -the setting. Any mark should be rubbed with the finger to assure that -it is not due to powder from the scratching agent; confusion may often -be caused in this way when the two substances are of nearly the same -hardness. - -The degrees of hardness of the gem-stones are given in Table VIII at -the end of the book. - - • • • • • - -It must not be overlooked that extreme hardness is compatible with -cleavability in certain directions intimately connected with the -crystalline structure; the property, in fact, characterizes many -mineral species of different degrees of hardness. Diamond can be split -in four directions parallel to the faces of the regular octahedron, -a property utilized by the lapidary for shaping a stone previous to -cutting it. Topaz cleaves with considerable ease at right angles to the -principal crystallographic axis. Felspar has two directions of cleavage -nearly at right angles to one another. The new gem-stone, kunzite, -needs cautious handling owing to the facility with which it splits in -two directions mutually inclined at about 70°. - -All stones are more or less brittle, and will be fractured by a -sufficiently violent blow, but the irregular surface of a fracture -cannot be mistaken for the brilliant flat surface given by a cleavage. -The cleavage is by no means induced with equal facility in the species -mentioned above. A considerable effort is required to split diamond, -but in the case of topaz or kunzite incipient cleavage in the shape of -flaws may be started if the stone be merely dropped on to a hard floor. - - - - - CHAPTER X - - ELECTRICAL CHARACTERS - - -The definite orientation of the molecular arrangement of crystallized -substances leads in many cases to attributes which vary with the -direction and are revealed by the electrical properties. If a -tourmaline crystal be heated in a gas or alcohol flame it becomes -charged with electricity, and, since it is at the same time a bad -conductor, static charges of opposite sign appear at the two ends. -Topaz shows similar characters, but in a lesser degree. Quartz, if -treated in the same way, shows charges of opposite sign on different -sides, but the phenomenon may be masked by intimate twinning and -consequent overlapping of the contrary areas. The phenomenon may -also be seen when the stones are cut. The most convenient method for -detecting the existence of the electrical charges is that devised -by Kundt. A powder consisting of a mixture of red lead and sulphur -is placed in a bellows arrangement and blown through a sieve at one -end on to the stone. Owing to the friction the particles become -electrified—red lead positively and sulphur negatively—and are -attracted by the charges of opposing sign, which will therefore be -betrayed by the colour of the dust at the corresponding spot. The -powder must be kept dry; otherwise a chemical reaction may occur -leading to the formation of lead sulphide, recognizable by its black -colour. Bücker has suggested as an alternative the use of sulphur, -coloured red with carmine, the negative element, and yellow lycopodium, -the positive element. - -Diamond, topaz, and tourmaline are powerful enough, when electrified -by friction with a cloth, to attract fragments of paper, the -electrification being positive. Amber develops considerable negative -electricity when treated in a similar manner. - -Diamond is translucent to the Röntgen (X) rays; glass, on the other -hand, is opaque to them, and this test distinguishes brilliants from -paste imitations. Diamond also, unlike glass, phosphoresces under the -influence of radium, a property characterizing also kunzite. - -It will be seen that the electrical characters, although of -considerable interest to the student, are, on account of their -limited application and difficulty of test, of little service for the -discrimination of gem-stones. - - - - - PART I—SECTION B - - THE TECHNOLOGY OF GEM-STONES - - - - - CHAPTER XI - - UNIT OF WEIGHT - - -The system in use for recording the weights of precious stones is -peculiar to jewellery. The unit, which is known as the carat, bears no -simple relation to any unit that has existed among European nations, -and indubitably has been introduced from the East. When man in early -days sought to record the weights of small objects, he made use of the -most convenient seeds or grains which were easily obtainable and were -at the same time nearly uniform in size. In Europe the smallest unit of -weight was the barley grain. Similarly in the East the seeds of some -leguminous tree were selected. Those of the locust-tree, _Ceratonia -siliqua_, which is common in the countries bordering the Mediterranean, -on the average weigh so nearly a carat that they almost certainly -formed the original unit. It is, indeed, from the Greek κεράτιον, -little horn, which refers to the shape of the pods, that the word carat -is derived. - -It is one of the eccentricities of the jewellery trade that precision -should not have been given to the unit of weight. Not only does it -vary at most of the trade centres in the world, but it is not even -always constant at each centre. The difference is negligible in the -case of single stones of ordinary size, but becomes a matter of serious -importance when large stones, or parcels of small stones, are bought -and sold, particularly when the stones are very costly. Attempts have -been made at various times to secure a uniform standard, but as yet -with only partial success. In 1871 the carat defined as the equivalent -of 0·20500 gram was suggested at a meeting of the principal jewellers -of Paris and London, and was eventually accepted in Paris, New York, -Leipzig, and Borneo. It has, however, recently been recognized that -in view of the gradual spread of the metric system of weights and -measures the most satisfactory unit is the metric carat of one-fifth -(0·2) gram. This has now been constituted the legal carat of France -and Belgium, and no doubt other countries will follow their example. -The carat weight obtaining in London weighs about 0·20530 gram, and -the approximate equivalents in the gram at other centres are as -follows:—Florence 0·19720, Madrid 0·20539, Berlin 0·20544, Amsterdam -0·20570, Lisbon 0·20575, Frankfort-on-Main 0·20577, Vienna 0·20613, -Venice 0·20700, and Madras 0·20735. The gram itself is inconveniently -large to serve as a unit for the generality of stones met with in -ordinary jewellery. - -The notation for expressing the sub-multiples of the carat forms -another curious eccentricity. Fractions are used which are powers of -the half: thus the half, the half of that, _i.e._ the quarter, and so -on down to the sixty-fourth, and the weight of a stone is expressed -by a series of fractions, _e.g._ 3½ ⅛ 1/64 carats. In the case of -diamond a single unreduced fraction to the base 64 is substituted in -place of the series of single fractions, and the weight of a stone is -stated thus, 4-40/64 carats. With the introduction of the metric -carat the more convenient and rational decimal notation would, of -course, be simultaneously adopted. - -[Illustration: Figs. 34-39.—Exact Sizes of Brilliants of various -Weights.] - -Figs. 34-39 illustrate the exact sizes of diamonds of certain weights, -when cut as brilliants. The sizes of other stones depends upon their -specific gravity, the weight varying as the volume multiplied by the -specific gravity. Quartz, for instance, has a low specific gravity and -would be perceptibly larger, weight for weight; zircon, on the other -hand, would be smaller. - -It has been found more convenient to select a smaller unit in the case -of pearls, namely, the pearl-grain, four of which go to the carat. - -Stencil gauges are in use for measuring approximately the weight in -carats of diamond brilliants and of pearls, which in both instances -must be unmounted. A more accurate method for determining the weight -of diamonds has been devised by Charles Moe, which is applicable to -either unmounted or mounted stones. By means of callipers, which read -to three-tenths of a millimetre, the diameter and the depth of the -stone are measured, and by reference to a table the corresponding -weight is found; allowance is made for the varying fineness of the -girdle, and, in the case of large stones, for the variation from a -strictly circular section. - - • • • • • - -Since this chapter was written the movement in favour of the metric -carat has made rapid progress, and this unit will soon have been -adopted as the legal standard all over the world, even in countries, -such as the British Isles and the United States, where the metric -system is not in use. The advantage of an international unit is too -obvious to need arguing. - - - - - CHAPTER XII - - FASHIONING OF GEM-STONES - - -Although many of the gem-stones have been endowed by nature with -brilliant lustrous faces and display scintillating reflections from -their surfaces, yet their form is never such as to reveal to full -perfection the optical qualities upon which their charm depends. -Moreover, the natural faces are seldom perfect; as a rule the stones -are broken either through some convulsion of the earth’s crust or in -course of extraction from the matrix in which they have lain, or they -are roughened by attrition against matter of greater hardness, or worn -by the prolonged action of water, or etched by solvents. Beautiful -octahedra of diamond or spinel have been mounted without further -embellishment, but even their appearance might have been much improved -at the lapidary’s hands. - -By far the oldest of the existing styles of cutting is the rounded -shape known as cabochon, a French word derived from the Latin _cabo_, -a head. In the days of the Roman Empire the softer stones were often -treated in this manner; such stones were supposed to be beneficial -to those suffering from short-sightedness, the reason no doubt being -that transparent stones when cut as a double cabochon formed a convex -lens. According to Pliny, Nero had an emerald thus cut, through -which he was accustomed to view the gladiatorial shows. This style of -cutting was long a favourite for coloured stones, such as emerald, -ruby, sapphire, and garnet, but has been abandoned in modern practice -except for opaque, semi-opaque, and imperfect stones. The crimson -garnet, which was at one time known by the name carbuncle, was so -systematically thus cut that the word has come to signify a red garnet -of this form. It was a popular brooch-stone with our grandmothers, but -is no longer in vogue. The East still retains a taste for stones cut -in the form of beads and drilled through the centre; the beads are -threaded together, and worn as necklaces. The native lapidaries often -improve the colour of pale emeralds by lining the hole with green paint. - -[Illustration: _PLATE V_ - -JEWELLERY DESIGNS] - -[Illustration: FIG. 40.—Double (Convex) Cabochon.] - -[Illustration: FIG. 41.—Simple Cabochon.] - -The cabochon form may be of three different kinds. In the first, the -double cabochon (Fig. 40), both the upper and the under sides of the -stones are curved. The curvature, however, need not be the same in -each case; indeed, it is usually markedly different. Moonstones and -starstones are generally cut very steep above and shallow underneath. -Occasionally a ruby or a sapphire is, when cut in this way, set with -the shallow side above, because the light that has penetrated into the -stone from above is more wholly reflected from a steep surface with -consequent increase in the glow of colour from the stone. Opals are -always cut higher on the exposed side, but the slope of the surface -varies considerably; they are generally cut steeply when required for -mounting in rings. Chrysoberyl cat’s-eyes are invariably cut with -curved bases in order to preserve the weight as great as possible. -The double cabochon form with a shallow surface underneath merges -into the second kind (Fig. 41) in which the under side is plane, the -form commonly employed for quartz cat’s-eyes, and occasionally also -for carbuncles. In this type the plane side is invariably mounted -downwards. In the third form (Fig. 42) the curvature of the under -surface is reversed, and the stone is hollowed out into a concave -shape. This style is reserved for dark stones, such as carbuncles, -which, if cut at all thick, would show very little colour. A piece of -foil is often placed in the hollow in order to increase the reflection -of light, and thus to heighten the colour effect. - -[Illustration: FIG. 42.—Double (Concavo-convex) Cabochon.] - -In early days it was supposed that the extreme hardness of diamond -precluded the possibility of fashioning it, and up to the fifteenth -century all that was done was to remove the gum-like skin which -disfigured the Indian stones and to polish the natural facets. -The first notable advance was made in 1475, when Louis de Berquem -discovered, as it is said quite by accident, that two diamonds if -rubbed together ground each other. With confident courage he essayed -the new art upon three large stones entrusted to him by Charles the -Bold, to the entire satisfaction of his patron. The use of wheels -or discs charged with diamond dust soon followed, but at first the -lapidaries evinced their victory over such stubborn material by -grinding diamond into divers fantastic shapes, and failed to realize -how much might be done to enhance the intrinsic beauty of the stones by -the means now at their disposal. The Indian lapidaries arrived at the -same discovery independently, and Tavernier found, when visiting the -country in 1665, a large number of diamond cutters actively employed. -If the stone were perfectly clear, they contented themselves with -polishing the natural facets; but if it contained flaws or specks, they -covered it with numerous small facets haphazardly placed. The stone was -invariably left in almost its original shape, and no effort was made to -improve the symmetry. - -[Illustration: FIG. 43.—Table Cut (top view).] - -[Illustration: FIG. 44.—Table Cut (side view).] - -For a long time little further progress was made, and even nearly a -century after Berquem the only regular patterns known to Kentmann, who -wrote in 1562, were the diamond-point and the diamond-table (Figs. -43-44). The former consisted of the natural octahedron facets ground -to regular shape, and was long employed for the minute stones which -were set in conjunction with large coloured stones in rings. The table -represented considerably greater labour. One corner of the regular -octahedron was ground down until the artificial facet thus produced was -half the width of the stone, while the opposite corner was slightly -ground. - -Still another century elapsed before the introduction of the rose -pattern, which comprised twenty-four triangular facets and a flat base -(Figs. 45-46), the stone being nearly hemispherical in shape. This -style is said to have been the invention of Cardinal Mazarin, but -probably he was the first to have diamonds of any considerable size cut -in this form. At the present day only tiny stones are cut as roses. - -[Illustration: FIG. 45.—Rose Cut (top view).] - -A few more years passed away, and at length at the close of the -seventeenth century diamond came by its own when Vincenzio Peruzzi, a -Venetian, introduced the brilliant form of cutting, and revealed for -the first time its amazing ‘fire.’ Except for minor changes this form -remains to this day the standard style for the shape of diamond, and -the word brilliant is commonly employed to denote diamond cut in this -way. So obviously and markedly superior is the style to all others -that upon its discovery the owners of large roses had them re-cut as -brilliants despite the loss in weight necessitated by the change. - -[Illustration: FIG. 46.—Rose Cut (side view).] - -The brilliant form is derived from the old table by increasing the -number of facets and slightly altering the angles pertaining to the -natural octahedron. In a perfect brilliant (Figs. 47-49) there are -altogether 58 facets, 33 above and 25 below the girdle, as the edge -separating the upper and lower portions of the stone is termed, which -are arranged in the following manner. Eight star-facets, triangular -in shape, immediately surround the large table-facet. Next come four -large templets or bezels, quadrilateral in form, arranged in pairs -on opposite sides of the table-facet, the four quoins or lozenges, -similar in shape, coming intermediately between them; in modern -practice, however, these two sets are identical in shape and size, -and there are consequently eight facets of the same kind instead of -two sets of four. The eight cross or skew facets and the eight skill -facets, in both sets the shape being triangular, form the boundary -of the girdle; modern brilliants usually have instead sixteen facets -of the same shape and size. The above 33 facets lie above the girdle -and form the crown of the stone. Immediately opposite and parallel to -the table is the tiny culet. Next to the latter come the four large -pavilion facets with the four quoins intermediately between them, -both sets being five-sided but nearly quadrilateral in shape; these -again are usually combined into eight facets of the same size. Eight -cross facets and eight skill facets, both sets, like those in the -crown, being triangular in shape, form the lower side of the girdle; -these also are generally united into a set of sixteen similar facets. -These 25 facets which lie below the girdle comprise the ‘pavilion,’ -or base of the stone. In a regular stone properly cut a templet is -nearly parallel to a pavilion, and an upper to a lower cross facet. The -contour of the girdle is usually circular, but occasionally assumes -less symmetrical shapes, as for instance in drop-stones or pendeloques, -and the facets are at the same time distorted. The number of facets may -with advantage be increased in the case of large stones. An additional -set of eight star facets is often placed round the culet, the total -number then being 66. It may be mentioned that the largest stone cut -from the Cullinan has the exceptional number of 74 facets. - -[Illustration: FIG. 47.—Brilliant Cut (top view).] - -[Illustration: FIG. 48.—Brilliant Cut (base view).] - -[Illustration: FIG. 49.—Brilliant Cut (side view).] - -In order to secure the finest optical effect certain proportions have -been found necessary. The depth of the crown must be one-half that of -the base, and therefore one-third the total depth of the stone, and the -width of the table must be slightly less than half that of the stone. -The culet should be quite small, not more in width than one-sixth of -the table; it is, in fact, not required at all except to avoid the -danger of the point splintering. The girdle should be as thin as is -compatible with strength sufficient to prevent chipping in the process -of mounting the stone; if it were left thick, the rough edge would be -visible by reflection at the lower facets, and would, especially if at -all dirty, seriously affect the quality of the stone. The shape of the -stone is largely determined by the sizes of the templets in the crown -and the pavilions in the base as compared with that of the table, or, -what comes to the same thing, by the inclinations at which they are cut -to that facet. If the table had actually half the width of the stone, -the angle[5] between it and a templet would be exactly half a right -angle or 45°; it is, however, made somewhat smaller, namely, about 40°. -A pavilion, being parallel to a templet, makes a similar angle with the -culet. The cross facets are more steeply inclined, and make an angle -of about 45° with the table or the culet, as the case may be. The star -facets, on the other hand, slant perceptibly less, and make an angle of -only about 26° with the table. A latitude of some 4° or 5° is possible -without seriously affecting the ‘fire’ of the stone. - -The object of the disposition of the facets on a brilliant is to assure -that all the light that enters the stone, principally by way of the -table, is wholly reflected from the base and emerges through the crown, -preferably by way of the inclined facets. A brilliant-cut diamond, if -viewed with the table between the observer and the light, appears quite -dark except for the small amount of light escaping through the culet. -Light should therefore fall on the lower facets at angles greater -than the critical angle of total-reflection, which for diamond is 24° -26´. The pavilions should be inclined properly at double this angle, -or 48° 52´, to the culet; but a ray that emerges at a pavilion in the -actual arrangement entered the table at nearly grazing incidence, and -the amount of light entering this facet at such acute perspective is -negligible. On the other hand, after reflection at the base light must, -in order to emerge, fall on the crown at less than the critical angle -of total-reflection. In Fig. 50 are shown diagrammatically the paths -of rays that entered the table in divers ways. The ray emerging again -at the table suffers little or no dispersion and is almost white, -but those coming out through the inclined facets are split up into -the rainbow effect, known as ‘fire,’ for which diamond is so famous. -It is in order that so much of the light entering by the table may -emerge through the inclined facets of the crown that the pavilions are -inclined at not much more than 40° to the culet. It might be suggested -that instead of being faceted the stone should be conically shaped, -truncated above and nearly complete below. The result would no doubt -be steadier, but, on the other hand, far less pleasing. It is the -ever-changing nuance that chiefly attracts the eye; now a brilliant -flash of purest white, anon a gleam of cerulean blue, waxing to -richest orange and dying in a crimson glow, all intermingled with the -manifold glitter from the surface of the stone. Absolute cleanliness is -essential if the full beauty of any stone is to be realized, but this -is particularly true of diamond. If the back of the stone be clogged -with grease and dirt, as so often happens in claw-set rings, light is -no longer wholly reflected from the base; much of it escapes, and the -amount of ‘fire’ is seriously diminished. - -[Illustration: FIG. 50.—Course of the Rays of Light passing through a -Brilliant.] - -Needless to state, lapidaries make no careful angular measurements -when cutting stones, but judge of the position of the facets entirely -by eye. It sometimes therefore happens that the permissible limits -are overstepped, in which event the stone is dead and may resist all -efforts to vivify it short of the heroic course of re-cutting it, too -expensive a treatment in the case of small stones. - -The factors that govern the properties of a brilliant-cut stone are -large colour-dispersion, high refraction, and freedom from any trace of -intrinsic colour. The only gem-stone that can vie with diamond in these -respects is zircon. Although it is rare to find a zircon naturally -without colour, yet many kinds are easily deprived of their tint by -the application of heat. A brilliant-cut zircon is, indeed, far from -readily distinguished by eye from diamond, and has probably often -passed as one, but it may easily be identified by its large double -refraction (cf. p. 41) and inferior hardness. The remaining colourless -stones, such as white sapphire, topaz, and quartz (rock-crystal), have -insufficient refractivity to give total-reflection at the base, and, -moreover, they are comparatively deficient in ‘fire.’ - -[Illustration: FIG. 51.—Step- or Trap-Cut (top view).] - -[Illustration: FIG. 52.—Step- or Trap-Cut (side view).] - -A popular style of cutting which is much in vogue for coloured stones -is the step- or trap-cut, consisting of a table and a series of -facets with parallel horizontal edges (Figs. 51-52) above and below -the girdle; in recent jewellery, however, the top of the stone is -often brilliant-cut. The contour may be oblong, square, lozenge, or -heart-shaped, or have less regular forms. The table is sometimes -slightly rounded. Since the object of this style is primarily to -display the intrinsic colour of the stone and not so much a brilliant -play of light from the interior, no attempt is made to secure -total-reflection at the lower facets. The stone therefore varies in -depth according to its tint; if dark, it is cut shallow, lest light -be wholly absorbed within, and the stone appear practically opaque, -but if light, it is cut deep, in order to secure fullness of tint. -Much precision in shape and disposition of the facets is not demanded, -and the stones are usually cut in such a way that, provided the -desired effect is obtained, the weight is kept as great as possible; -we may recall that stones are sold by weight. In considering what -will be the optical effect of any particular shape, regard must be -had to the effective colour of the transmitted light. For instance, -although sapphire and ruby belong to the same species and have the -same refractive indices, yet, since the former transmits mainly blue -and the latter red light, they have for practical purposes appreciably -different indices, and lapidaries find it therefore possible to cut -the base of ruby thicker than that of sapphire, and thus keep the -weight greater. It is instructive too what can be done with the most -unpromising material by the exercise of a little ingenuity. Thus Ceylon -sapphires are often so irregularly coloured that considerable skill -is called for in cutting them. A stone may, for instance, be almost -colourless except for a single spot of blue; yet, if the stone be -cut steeply and the spot be brought to the base, the effect will be -precisely the same as if the stone were uniformly coloured, because all -the light emerging from the stone has passed through the spot at the -base and therefore been tinted blue. - -The mechanism employed in the fashioning of gem-stones is simple in -character, and comprises merely metal plates or wheels for slitting, -and discs or laps for grinding and polishing the stones, the former -being set vertically and rotated about horizontal spindles, and the -latter set horizontally and rotated about vertical spindles. Mechanical -power is occasionally used for driving both kinds of apparatus, but -generally, especially in slitting and in delicate work, hand-power is -preferred. In the East native lapidaries make use of vertical wheels -(Plate XIII) also for grinding and polishing stones, which explains why -native-cut stones never have truly plane facets; it will be noticed -from the picture that a long bow is used to drive the spindle. - -Owing to the unique hardness of diamond it can be fashioned only by -the aid of its own powder. The process differs therefore materially -from the cutting of the remaining gem-stones, and will be described -separately. Indeed, so different are the two classes of work that firms -seldom habitually undertake both. - -The discovery of the excellent cleavage of diamond enormously reduced -the labour of cutting large stones. A stone containing a bad flaw may -be split to convenient shape in as many minutes as the days or even -weeks required to grind it down. The improvement in the appliances and -the provision of ample mechanical power has further accelerated the -process and reduced the cost. Two years were occupied in cutting the -diamond known as the Pitt or Regent, whereas in only six months the -colossal Cullinan was shaped into two large and over a hundred smaller -stones with far less loss of material. - -Although the brilliant form was derived from the regular octahedron, it -by no means follows that, because diamond can be cleaved to the latter -form, such is the initial step in fashioning the rough mass. The aim of -the lapidary is to cut the largest possible stone from the given piece -of rough, and the finished brilliant usually bears no relation whatever -to the natural octahedron. The cleavage is utilized only to free the -rough of an awkward and useless excrescence, or of flaws. Although the -octahedron is one of the common forms in which diamond is found, it is -rarely regular, and oftener than not one of the larger faces is made -the table. - -The old method, which is still in use, for roughly fashioning diamonds -is that known as bruting, from the French word, _brutage_, for the -process, or as shaping. Two stones of about the same size are selected, -and are firmly attached by means of a hard cement to the ends of -two holders, which are held one in each hand, and rubbed hard, one -against the other, until surfaces of the requisite size are developed -on each stone. During the process the stones are held over a small -box, which catches the precious powder. A fine sieve at the bottom -of the box allows the powder to fall through into a tray underneath, -but holds back anything larger. By means of two vertical pins placed -one on each side of the box the holders are retained more easily in -the desired position, and the work is thrown mainly on the thumbs. -This work continued day after day has a very disfiguring effect upon -the hands despite the thick gloves that are worn to protect them; the -skin of the thumbs grows hard and horny, and the first and second -fingers become swollen and distorted. When the surfaces have thus been -formed, the stone is handed to the polisher, who works them into the -correct shape and afterwards polishes them, the stone passing backwards -and forwards several times between the cutter and the polisher. The -table, four templets, culet and four pavilions are first formed and -polished, so that the table has a square shape. Next the quoins are -developed and polished, and finally the small facets are polished on, -not being shaped first. In modern practice the process of bruting has -been modified in some cases by the introduction of machinery, and the -facets are ground on, with considerable improvement in the regularity -of their size and disposition, and reduction in the amount of polishing -required. Moreover, to obviate the loss of material resulting from -continued grinding, large stones are first sliced by means of -rapidly-revolving copper wheels charged with diamond powder. - -The laps used for polishing diamonds are made of a particular kind -of soft iron, which is found to surpass any other metal in retaining -the diamond powder. They are rotated at a high rate of speed, which -is about 2000 to 2500 revolutions a minute, and the heat developed by -the friction at this speed is too great for a cement to be used; a -solder or fusible alloy, composed of one part tin to three parts lead, -therefore takes its place. The solder is held in a hollow cup of brass -which is from its shape called a ‘dop,’ an old Dutch word meaning -shell. Its external diameter is ordinarily about 1½ in. (4 cm.), but -larger dops are, of course, used for large stones. A stout copper stalk -is attached to the bottom of the dop; it is visible in the view of the -dop shown at _e_ on Plate VI, and two slabs of solder are seen lying in -front of the dop. The dop containing the solder is placed in the midst -of a non-luminous flame and heated until the solder softens, when it is -removed by means of the small tongs, _c_, and placed upright on a stand -such as that shown at _a_. The long tongs, _d_, are used for shaping -the solder into a cone at the apex of which the diamond is placed. The -solder is worked well over the stone so that only the part to undergo -polishing is exposed. A diamond in position is shown at _f_. The top of -the stand is saucer-shaped to catch the stone should it accidentally -fall off the dop, and to prevent pieces of solder falling on the hand. -While still hot, the dop with the diamond in position on the solder is -plunged into cold water in order to cool it. The fact that the stone -withstands this drastic treatment is eloquent testimony to its good -thermal conductivity; other gem-stones would promptly split into -fragments. It may be remarked that so high is the temperature at which -diamond burns that it may be placed in the gas flame without any fear -of untoward results. The dop is now ready for attachment to an arm -such as that shown at _b_; the stalk of the dop is placed in a groove -running across the split end of the arm, and is gripped tight by means -of a screw worked by the nut which is visible in the picture. - -[Illustration: _PLATE VI_ - -APPLIANCES USED FOR POLISHING DIAMONDS.] - -[Illustration: _PLATE VII_ - -POLISHING DIAMONDS] - -Four such arms, each with a dop, are used with the polishing lap (Plate -VII), and each stands on two square legs on the bench. Pins, _p_, in -pairs are fixed to the bench to prevent the arms being carried round by -the friction; one near the lap holds the arm not far from the dop, and -the other engages in a strong metal tongue, which is best seen at the -end of the arm _b_ on Plate VI. Though the arm, which is made of iron, -is heavy, yet for polishing purposes it is insufficient, and additional -lead weights are laid on the top of it, as in the case of the arm -at the back on Plate VII. The copper stalk is strong, yet flexible, -and can be bent to suit the position of the facet to be polished; on -Plate VII the dops _a_ and _b_ are upright, but the other two are -inclined. In addition to the powder resulting from bruting, boart, -_i.e._ diamonds useless for cutting, are crushed up to supply polishing -material, and a little olive oil is used as a lubricant. Owing to the -friction so much heat is developed that even the solder would soften -after a time, and therefore, as a precaution, the dop is from time -to time cooled by immersion in water. The stone has constantly to be -re-set, about six being the maximum even of the tiny facets near the -girdle that can be dealt with by varying the inclination of the dop. -As the work approaches completion the stone is frequently inspected, -lest the polishing be carried too far for the development of the proper -amount of ‘fire.’ When finished, the stones are boiled in sulphuric -acid to remove all traces of oil and dirt. - -The whole operation is evidently rough and ready in the extreme; but -such amazing skill do the lapidaries acquire, that even the most -careful inspection by eye alone would scarce detect any want of proper -symmetry in a well-cut stone. - -The fashioning of coloured stones, as all the gem-stones apart from -diamond are termed in the jewellery trade, is on account of their -inferior hardness a far less tedious operation. They are easily -slit, for which purpose a vertical wheel (Plate VIII) made of soft -iron is used; it is charged with diamond dust and lubricated with -oil, generally paraffin. When slit to the desired size, the stone is -attached to a conveniently shaped holder by means of a cement, the -consistency of which varies with the hardness of the stone. It is -set in the cement in such a way that the plane desired for the table -facet is at right angles to the length of the holder, and the whole of -the upper part or crown is finished before the stone is removed from -the cement. The lower half or base is treated in a similar manner. -Thus in the process of grinding and polishing the stone is only once -re-set; as was stated above, diamond demands very different treatment. -Again, all coloured stones are ground down without any intermediate -operation corresponding to bruting. The holder is merely held in the -hand, but to maintain its position more exactly its other end, -which is pointed, is inserted in one of the holes that are pierced at -intervals in a vertical spindle placed at a convenient distance from -the lap (Plate VIII), which one depending upon the inclination of the -facet to be formed. For hard stones, such as ruby and sapphire, diamond -powder is generally used as the abrasive agent, while for the softer -stones emery, the impure corundum, is selected; in recent years the -artificially prepared carborundum, silicide of carbon corresponding to -the formula CSi, which is harder than corundum, has come into vogue -for grinding purposes, but it is unfortunately useless for slitting, -because it refuses to cling to the wheel. To efface the scratches left -by the abrasive agent and to impart a brilliant polish to the facets, -material of less hardness, such as putty-powder, pumice, or rouge, is -employed; in all cases the lubricant is water. The grinding laps are -made of copper, gun-metal, or lead; and pewter or wooden laps, the -latter sometimes faced with cloth or leather, are used for polishing. -As a general rule, the harder the stone the greater the speed of the -lap. - -[Illustration: _PLATE VIII_ - -SLITTING COLOURED STONES - -POLISHING COLOURED STONES] - -[Illustration: _PLATE IX_ - -FACETING MACHINE] - -As in the case of diamond, the lapidary judges of the position of the -facet entirely by eye and touch, but a skilled workman can develop -a facet very close to the theoretical position. During recent years -various devices have been invented to enable him to do his work with -greater facility. A machine of this kind is illustrated on Plate IX. -The stone is attached by means of cement to the blunt end, _d_, of -the holder, _b_, which is of the customary kind, while the other end -is inserted in a hole in a wooden piece, _a_, which is adjustable in -height by means of the screw above it. The azimuthal positions of the -facets are arranged by means of the octagonal collar, _c_, the sides -of which are held successively in turn against the guide, _e_. The -stand itself is clamped to the bench. The machine is, however, little -used except for cheap stones, because it is too accurate and leads to -waste of material. Stones are sold by weight, and so long as the eye is -satisfied, no attempt is made to attain to absolute symmetry of shape. - -The pictures on Plates X-XIII illustrate lapidaries’ workshops in -various parts of the world. The first two show an office and a workshop -situated in Hatton Garden, London; in the former certain of the staff -are selecting from the parcels stones suitable for cutting. The third -depicts a more primitive establishment at Ekaterinburg in the Urals. -The fourth shows a typical French family—_père_, _mère_, _et fils_—in -the Jura district, all busily engaged; on the table will be noticed a -faceting machine of the kind described above. In the fifth picture a -native lapidary in Calcutta is seen at work with the driving bow in his -right, and the stone in his left, hand. - -A curious difference exists in the systems of charging for cutting -diamonds and coloured stones. The cost of cutting the latter is -reckoned by the weight of the finished stone, the rate varying from -1s. to 8s. a carat according to the character of the stone and the -difficulty of the work; while in the case of diamonds, on the other -hand, the weight of the rough material determines the cost, the rate -being about 10s. to 40s. a carat according to the size, which on the -average is equivalent to about 30s. to 120s. a carat calculated on -the weight of the finished stone. The reason of the distinction is -obviously because the proper proportions in a brilliant-cut diamond -must be maintained, whatever be the loss in weight involved; in -coloured stones the shape is not of such primary importance. - -[Illustration: _PLATE X_ - -LAPIDARY’S WORKSHOP AND OFFICE IN ENGLAND] - -[Illustration: _PLATE XI_ - -LAPIDARY’S WORKSHOP IN RUSSIA] - -When finished, the stone finds its way with others akin to it to the -manufacturing jeweller’s establishment, where it is handed to the -setter, who mounts it in a ring, necklace, brooch, or whatever article -of jewellery it is intended for. The metal used in the groundwork of -the setting is generally gold, but platinum is also employed where an -unobtrusive and untarnishable metal is demanded, and silver finds a -place in cheaper jewellery, although it is seriously handicapped by -its susceptibility to the blackening influence of the sulphurous fumes -present in the smoke-laden atmosphere of towns. The stone may be either -embedded in the metal or held by claws. The former is by far the safer, -but the latter the more elegant, and it has the advantage of exposing -the stone _à jour_, to use the French jewellers’ expression, so that -its genuineness is more evidently testified. It is very important -that the claw setting be periodically examined, lest the owner one -day experience the mortification of finding that a valuable stone has -dropped out; gold, owing to its softness, wears away in course of time. - -Up to quite recent years modern jewellery was justly open to the -criticism that it was lacking in variety, that little attempt was made -to secure harmonious association in either the colour or the lustre of -the gem-stones, and that the glitter of the gold mount was frequently -far too obtrusive. Gold consorts admirably with the rich glow of -ruby, but is quite unsuited to the gleaming fire of a brilliant. Where -the metal is present merely for the mechanical purpose of holding the -stones in position, it should be made as little noticeable as possible. -The artistic treatment of jewellery is, however, receiving now adequate -attention in the best Paris and London houses. Some recent designs are -illustrated on Plates IV and V. - -[Illustration: _PLATE XII_ - -FRENCH FAMILY CUTTING STONES] - -[Illustration: _PLATE XIII_ - -INDIAN LAPIDARY] - - - - - CHAPTER XIII - - NOMENCLATURE OF PRECIOUS STONES - - -The names in popular use for the principal gem-stones may be traced -back to very early times, and, since they were applied long before -the determinative study of minerals had become a science, their -significance has varied at different dates, and is even now far from -precise. No ambiguity or confusion could arise if jewellers made use -of the scientific names for the species, but most of them are unknown -or at least unfamiliar to those unversed in mineralogy, and to banish -old-established names is undesirable, even if the task were not -hopeless. The name selected for a gem-stone may have a very important -bearing on its fortunes. When the love-sick Juliet queried ‘What’s -in a name?’ her mind was wandering far from jewels; for them a name -is everything. The beautiful red stones that accompany the diamond -in South Africa were almost a drug in the market under their proper -title—garnet, but command a ready sale under the misnomer ‘Cape-ruby.’ -To many minds there is a subtle satisfaction in the possession of a -stone which is assumed to be a sort of ruby that would be destroyed by -the knowledge that the stone really belonged to the cinderella species -of gem-stones—the despised garnet. For similar reasons it was deemed -advisable to offer the lustrous green garnet found some thirty and odd -years ago in the Ural Mountains as ‘olivine,’ not a happy choice since -their colour is grass- rather than olive-green, apart from the fact -that the term is in general use in science for the species known in -jewellery as peridot. - -The names employed in jewellery are largely based upon the colour, the -least reliable from a determinative point of view of all the physical -characters of gem-stones. Qualifying terms are employed to distinguish -stones of obviously different hardness. ‘Oriental’ distinguishes -varieties of corundum, but does not imply that they necessarily came -from the East; the finest gem-stones originally reached Europe by that -road, and the hardest coloured stones consequently received that term -of distinction. - -Nearly all red stones are grouped under the name ruby, which is derived -from a Latin word, _ruber_, meaning red, or under other names adapted -from it, such as rubellite, rubicelle. It is properly applied to red -corundum; ‘balas’ ruby is spinel, which is associated with the true -ruby at the Burma mines and is similar in appearance to it when cut, -and ‘Cape’ ruby, is, as has been stated above, a garnet from South -Africa. Rubellite is the lovely rose-pink tourmaline, fine examples of -which have recently been discovered in California, and rubicelle is a -less pronouncedly red spinel. Sapphire is by far the oldest and one -of the most interesting of the words used in the language of jewels. -It occurs in Hebrew and Persian, ancient tongues, and means blue. -It was apparently employed for lapis lazuli or similar substance, -but was transferred to the blue corundum upon the discovery of this -splendid stone. Oblivious of the real meaning of the word, jewellers -apply it in a quasi-generic sense to all the varieties of corundum -with the exception of the red ruby, and give vent to such incongruous -expressions as ‘white sapphire,’ ‘yellow sapphire’; it is true such -stones often contain traces of blue colour, but that is not the reason -of the terms. ‘Brazilian’ sapphire is blue tourmaline, a somewhat rare -tint for this species. The curious history of the word topaz will be -found below in the chapter dealing with the species of that name. It -has always denoted a yellow stone, and at the present day is applied -by jewellers indiscriminately to the true topaz and citrine, the -yellow quartz, the former, however, being sometimes distinguished by -the prefix ‘Brazilian.’ ‘Oriental’ topaz is corundum, and ‘occidental’ -topaz is a term occasionally employed for the yellow quartz. Emerald, -which means green, was first used for chrysocolla, an opaque greenish -stone (p. 288), but was afterwards applied to the priceless green -variety of beryl, for which it is still retained. ‘Oriental’ emerald -is corundum, ‘Brazilian’ emerald in the eighteenth century was a -common term for the green tourmaline recently introduced to Europe, -and ‘Uralian’ emerald has been tentatively suggested for the green -garnet more usually known as ‘olivine.’ Amethyst is properly the violet -quartz, but with the prefix ‘oriental’ it is also applied to violet -corundum, though some jewellers use it for the brilliant quartz, with -purple and white sectors, from Siberia. Almandine, which is derived -from the name of an Eastern mart for precious stones, has come to -signify a stone of columbine-red hue, principally garnet, but with -suitable qualification corundum and spinel also. - -The nomenclature of jewellery tends to suggest relations between the -gem-stones for which there is no real foundation, and to obscure -the essential identity, except from the point of view of colour, of -sapphire and ruby, emerald and aquamarine, cairngorm and amethyst. - - - - - CHAPTER XIV - - MANUFACTURED STONES - - -The initial step in the examination of a crystallized substance is -to determine its physical characters and to resolve it by chemical -analysis into its component elements; the final, and by far the -hardest, step is to build it up or synthetically prepare it from its -constituents. Unknown to the world at large, work of the latter kind -has long been going on within the walls of laboratories, and as the -advance in knowledge placed in the hands of experimenters weapons more -and more comparable with those wielded by nature, their efforts have -been increasingly successful. So stupendous, however, are the powers -of nature that the possibility of reproducing, by human agency, the -treasured stones which are extracted from the earth in various parts -of the globe at the cost of infinite toil and labour has always been -derided by those ignorant of what had already been accomplished. Great, -therefore, was the consternation and the turmoil when concrete evidence -that could not be gainsaid showed that man’s restless efforts to -bridle nature to his will were not in vain, and congresses of all the -high-priests of jewellery were hastily convened to ban such unrighteous -products, with what ultimate success remains to be seen. - -Crystallization may be caused in four different ways, of which the -second alone has as yet yielded stones large enough to be cut— - -1. By the separation of the substance from a saturated solution. In -nature the solvent may not be merely hot water, or water charged with -an acid, but molten rock, and the temperature and the pressure may be -excessively high. - -2. By the solidification of the liquefied substance upon cooling. Ice -is a familiar example of this type. - -3. By the sublimation of the vapour of the substance, which means the -direct passage from the vapour to the solid state without traversing -the usually intervening liquid state. It is usually the most difficult -of attainment of the four methods; the most familiar instance is snow. - -4. By the precipitation of the substance from a solution when set free -by chemical action. - -Other things being equal, the simpler the composition the greater is -the ease with which a substance may be expected to be formed; for, -instead of one complex substance, two or more different substances -may evolve, unless the conditions are nicely arranged. Attempts, -for instance, to produce beryl might result instead in a mixture of -chrysoberyl, phenakite, and quartz. - -By far the simplest in composition of all the precious stones is -diamond, which is pure crystallized carbon; but its manufacture is -attended by well-nigh insuperable difficulties. If carbon be heated -in air, it burns at a temperature well below its melting point; -moreover, unless an enormously high pressure is simultaneously applied, -the product is the other form of crystallized carbon, namely, the -comparatively worthless graphite. Moissan’s interesting course of -experiments were in some degree successful, but the tiny diamonds were -worthless as jewels, and the expense involved in their manufacture was -out of all proportion to any possible commercial value they might have. - -Next to diamond the simplest substances among precious stones are -quartz (crystallized silica) and corundum (crystallized alumina). The -crystallization of silica has been effected in several ways, but the -value in jewellery of quartz, even of the violet variety, amethyst, is -not such as to warrant its manufacture on a commercial scale. Corundum, -on the other hand, is held in high esteem; rubies and sapphires, of -good colour and free from flaws, have always commanded good prices. The -question of their production by artificial means has therefore more -than academic interest. - -Ever since the year 1837, when Gaudin produced a few tiny flakes, -French experimenters have steadily prosecuted their researches in the -crystallization of corundum. Frémy and Feil, in 1877, were the first -to meet with much success. A portion of one of their crucibles lined -with glistening ruby flakes is exhibited in the British Museum (Natural -History). - -[Illustration: FIG. 53.—Verneuil’s Inverted Blowpipe.] - -In 1885 the jewellery market was completely taken by surprise by the -appearance of red stones, emanating, so it is alleged, from Geneva; -having the physical characters of genuine rubies, they were accepted -as, and commanded the prices of, the natural stones. It was eventually -discovered that they had resulted from the fusion of a number of -fragments of natural rubies in the oxy-hydrogen flame. The original -colour was driven off at that high temperature, but was revived by the -previous addition of a little bichromate of potassium. Owing to the -inequalities of growth, the cracks due to rapid cooling, the inclusion -of air-bubbles, often so numerous as to cause a cloudy appearance, -and, above all, the unnatural colour, these reconstructed stones, as -they are termed, were far from satisfactory, but yet they marked such -an advance on anything that had been accomplished before that for some -time no suspicion was aroused as to their being other than natural -stones. - -A notable advance in the synthesis of corundum, particularly of ruby, -was made in 1904, when Verneuil, who had served his apprenticeship -to science under the guidance of Frémy, invented his ingenious -inverted form of blowpipe (Fig. 53), which enabled him to overcome the -difficulties that had baffled earlier investigators, and to manufacture -rubies vying in appearance after cutting with the best of nature’s -productions. The blowpipe consisted of two tubes, of which the upper, -_E_, wide above, was constricted below, and passing down the centre of -the lower, _F_, terminated just above the orifice of the latter in -a fine nozzle. Oxygen was admitted at _C_ through the plate covering -the upper end of the tube, _E_. A rod, which passed through a rubber -collar in the same plate, supported inside the tube, _E_, a vessel, -_D_, and at the upper end terminated in a small plate, on which was -fixed a disc, _B_. The hammer, _A_, when lifted by the action of an -electromagnet and released, fell by gravity and struck the disc. The -latter could be turned about a horizontal axis placed eccentrically, -so that the height through which the hammer fell and the consequent -force of the blow could be regulated. The rubber collar, which was -perfectly gas-tight, held the rod securely, but allowed the shocks to -be transmitted to the vessel, _D_, an arrangement of guides maintaining -the slight motion of the vessel strictly vertical. This vessel, which -carried the alumina powder used in the manufacture of the stone, had as -its base a cylindrical sieve of fine mesh. The succession of rapid taps -of the hammer caused a regular feed of powder down the tube, the amount -being regulated by varying the height through which the hammer fell. -Hydrogen or coal-gas was admitted at _G_ into the outer tube, _F_, and -in the usual way met the oxygen just above the orifice, _L_. To exclude -irregular draughts, the flame was surrounded by a screen, _M_, which -was provided with a mica window, and a water-jacket, _K_, protected the -upper part of the apparatus from excessive heating. - -[Illustration: FIG. 54.—‘Boule,’ or Pear-shaped Drop.] - -The alumina was precipitated from a solution of pure ammonia—alum, -(NH_{4})_{2}SO_{4}.Al_{2}(SO_{4})_{3}.24H_{2}O, in distilled water -by the addition of pure ammonia, sufficient chrome-alum also being -dissolved with the ammonia-alum to furnish about 2½ per cent. of -chromic oxide in the resulting stone. The powder, carefully prepared -and purified, was placed, as has been stated above, in the vessel, -_D_, and on reaching the flame at the orifice it melted, and fell -as a liquid drop, _N_, upon the pedestal, _P_, which was formed of -previously fused alumina. This pedestal was attached by a platinum -sleeve to an iron rod, _Q_, which was provided with the necessary screw -adjustments, _R_ and _S_, for centring and lowering it as the drop -grew in size. Great care was exercised to free the powder from any -trace of potassium, which, if present, imparted a brownish tinge to -the stone. The pressure of the oxygen, low initially both to prevent -the pedestal from melting, and to keep the area of the drop in contact -with the pedestal as small as possible, because otherwise flaws tended -to start on cooling, was gradually increased until the flame reached -the critical temperature which kept the top of the drop melted, but -not boiling. The supply of powder was at the same time carefully -proportioned to the pressure. The pedestal, _P_, was from time to time -lowered, and the drop grew in the shape of a pear (Fig. 54), the apex -of which was downwards and adhered to the pedestal by a narrow stalk. -As soon as the drop reached the maximum size possible with the size of -the flame, the gases were sharply and simultaneously cut off. After ten -minutes or so the drop was lowered from the chamber, _M_, by the screw, -_S_, and when quite cold was removed from the pedestal. - -Very few changes have been made in the method when adapted to -commercial use. Coal-gas has, however, entirely replaced the -costly hydrogen, and the hammer is operated by a cam instead of an -electromagnet, while, as may be seen from the view of a gem-stone -factory (Plate XIV), a number of blowpipes are placed in line so that -their cams are worked by the same shaft, _a_. The fire-clay screen, -_b_, surrounding the flame is for convenience of removal divided into -halves longitudinally, and a small hole is left in front for viewing -the stone during growth, a red glass screen, _c_, being provided in -front to protect the eyes from the intense glare. Half the fire-clay -screen of the blowpipe in the centre of the Plate has been removed to -show the arrangement of the interior. The centring and the raising -and lowering apparatus, _d_, have been modified. The process is so -simple that one man can attend to a dozen or so of these machines, and -it takes only one hour to grow a drop large enough to be cut into a -ten-carat stone. - -[Illustration: _PLATE XIV_ - -BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES] - -The drops, unless the finished stone is required to have a similar -pear shape, are divided longitudinally through the central core into -halves, which in both shape and orientation are admirably suited to the -purposes of cutting; as a general rule, the drop splits during cooling -into the desired direction of its own accord. - -[Illustration: FIG. 55.—Bubbles and Curved Striæ in Manufactured Ruby.] - -Each drop is a single crystalline individual, and not, as might have -been anticipated, an alumina glass or an irregular aggregation of -crystalline fragments, and, if the drop has cooled properly, the -crystallographic axis is parallel to the core of the pear. The cut -stone will therefore have not only the density and hardness, but also -all the optical characters—refractivity, double refraction, dichroism, -etc.—pertaining to the natural species, and will obey precisely the -same tests with the refractometer and the dichroscope. Were it not for -certain imperfections it would be impossible to distinguish between the -stones formed in Nature’s vast workshop and those produced within the -confines of a laboratory. The artificial stones, however, are rarely, -if ever, free from minute air-bubbles (Fig. 55), which can easily be -seen with an ordinary lens. Their spherical shape differentiates them -from the plane-sided cavities not infrequently visible in a natural -stone (Fig. 56). Moreover, the colouring matter varies slightly, but -imperceptibly, in successive shells, and consequently in the finished -stone a careful eye can discern the curved striations (Fig. 55) -corresponding in shape to the original shell. In a natural stone, on -the other hand, although zones of different colours or varying shades -are not uncommon, the resulting striations are straight (Fig. 56), -corresponding to the plane faces of the original crystal form. By -sacrificing material it might be possible to cut a small stone free -from bubbles, but the curved striations would always be present to -betray its origin. - -[Illustration: FIG. 56.—Markings in Natural Ruby.] - -The success that attended the manufacture of ruby encouraged efforts to -impart other tints to crystallized alumina. By reducing the percentage -amount of chromic oxide, pink stones were turned out, in colour not -unlike those Brazilian topazes, the original hue of which has been -altered by the application of heat. These artificial stones have -therefore been called ‘scientific topaz’; of course, quite wrongly, -since topaz, which is properly a fluo-silicate of aluminium, is quite a -different substance. - -Early attempts made to obtain the exquisite blue tint of the true -sapphire were frustrated by an unexpected difficulty. The colouring -matter, cobalt oxide, was not diffused evenly through the drop, but -was huddled together in splotches, and it was found necessary to add a -considerable amount of magnesia as a flux before a uniform distribution -of colour could be secured. It was then discovered that, despite the -colour, the stones had the physical characters, not of sapphire, but -of the species closely allied to it, namely, spinel, aluminate of -magnesium. By an unsurpassable effort of nomenclature these blue stones -were given the extraordinary name of ‘Hope sapphire,’ from fanciful -analogy with the famous blue diamond which was once the pride of the -Hope collection. A blue spinel is occasionally found in nature, but the -actual tint is somewhat different. These manufactured stones have the -disadvantage of turning purple in artificial light. By substituting -lime for magnesia as a flux, Paris, a pupil of Verneuil’s, produced -blue stones which were not affected to the same extent. The difficulty -was at length overcome at the close of 1909, when Verneuil, by -employing as tinctorial agents 0·5 per cent. of titanium oxide and 1·5 -per cent. of magnetic iron oxide, succeeded in producing blue corundum; -it, however, had not quite the tint of sapphire. Stones subsequently -manufactured, which were better in colour, contained about 0·12 per -cent. of titanium oxide, but no iron at all. - -By the addition to the alumina of a little nickel oxide and vanadium -oxide respectively, yellow and yellowish green corundums have been -obtained. The latter have in artificial light a distinctly reddish -hue, and have therefore been termed ‘scientific alexandrite’; of -course, quite incorrectly, since the true alexandrite is a variety of -chrysoberyl, aluminate of beryllium, a very different substance. - -If no colouring matter at all be added and the alum be free from -potash, colourless stones or white sapphires are formed, which pass -under the name ‘scientific brilliant.’ It is scarcely necessary to -remark that they are quite distinct from the true brilliant, diamond. - -The high prices commanded by emeralds, and the comparative success that -attended the reconstruction of ruby from fragments of natural stones, -suggested that equal success might follow from a similar process with -powdered beryl, chromic oxide being used as the colouring agent. The -resulting stones are, indeed, a fair imitation, being even provided -with flaws, but they are a beryl glass with lower specific gravity and -refractivity than the true beryl, and are wrongly termed ‘scientific -emerald.’ Moreover, recently most of the stones so named on the market -are merely green paste. - -It is unfortunate that the real success which has been achieved in the -manufacture of ruby and sapphire should be obscured by the ill-founded -claims tacitly asserted in other cases. - -At the time the manufactured ruby was a novelty it fetched as much as -£6 a carat, but as soon as it was discovered that it could easily -be differentiated from the natural stone, a collapse took place, and -the price fell abruptly to 30s., and eventually to 5s. and even 1s. -a carat. The sapphires run slightly higher, from 2s. to 7s. a carat. -The prices of the natural stones, which at first had fallen, have now -risen to almost their former level. The extreme disparity at present -obtaining between the prices of the artificial and the natural ruby -renders the fraudulent substitution of the one for the other a great -temptation, and it behoves purchasers to beware where and from whom -they buy, and to be suspicious of apparently remarkable bargains, -especially at places like Colombo and Singapore where tourists abound. -It is no secret that some thousands of carats of manufactured rubies -are shipped annually to the East. _Caveat emptor._ - - - - - CHAPTER XV - - IMITATION STONES - - -The beryl glass mentioned in the previous chapter marks the transition -stage between manufactured stones which in all essential characters -are identical with those found in nature, and artificial stones which -resemble the corresponding natural stone in outward appearance only. In -a sense both sorts may be styled artificial, but it would be misleading -to confound them under the same appellation. - -Common paste,[6] which is met with in drapery goods and cheap ornaments -in general—hat-pins, buckles, and so forth—is composed of ordinary -crown-glass or flint-glass, the refractive indices being about 1·53 and -1·63 respectively. The finest quality, which is used for imitations of -brilliants, is called ‘strass.’ It is a dense lead flint-glass of high -refraction and strong colour-dispersion, consisting of 38·2 per cent. -of silica, 53·3 red lead (oxide of lead), and 7·8 potassium carbonate, -with small quantities of soda, alumina, and other substances. How -admirable these imitations may be, a study of the windows of a shop -devoted to such things will show. Unfortunately the addition of -lead, which is necessary for imparting the requisite refraction and -‘fire’ to the strass, renders the stones exceedingly soft. All glass -yields to the file, but strass stones are scratched even by ordinary -window-glass. If worn in such a way that they are rubbed, they -speedily lose the brilliance of their polish, and, moreover, they are -susceptible to attack by the sulphurous fumes present in the smoky air -of towns, and turn after a time a dirty brown in hue. When coloured -stones are to be imitated, small quantities of a suitable metallic -oxide are fused with the glass; cobalt gives rise to a royal-blue tint, -chromium a ruby red, and manganese a violet. Common paste is not highly -refractive enough to give satisfactory results when cut as a brilliant, -and the bases are therefore often coated with quicksilver, or, in the -case of old jewellery, covered with foil in the setting, in order to -secure more complete reflection from the interior. The fashioning of -these imitation stones is easy and cheap. Being moulded, they do not -require cutting, and the polishing of the facets thus formed is soon -done on account of the softness of the stones. - -A test with a file readily differentiates paste stones from the natural -stones they pretend to be. Being necessarily singly refractive, they -are, of course, lacking in dichroism, and their refractivity seldom -accords even approximately with that of the corresponding natural stone. - -In order to meet the test for hardness the doublet was devised. Such -a stone is composed of two parts—the crown consisting of colourless -quartz or other inexpensive real and hard stone, and the base being -made up of coloured glass. When the imitation, say of a sapphire, is -intended to be more exact, the crown is made of a real sapphire, but -one deficient in colour, the requisite tint being obtained from the -paste forming the under part of the doublet. In case the base should -also be tested for hardness the triplet has been devised. In this the -base is made of a real stone also, and the coloured paste is confined -to the girdle section, where it is hidden by the setting. Sapphires and -emeralds of indifferent colour are sometimes slit across the girdle; -the interior surfaces are polished, and colouring matter is introduced -with the cement, generally Canada balsam, which is used to re-unite the -two portions of the stone together. All such imitations may be detected -by placing the stone in oil, when the surfaces separating the portions -of the composite stone will be visible, or the binding cement may be -dissolved by immersing the stone, if unmounted, in boiling water, or in -alcohol or chloroform, when the stone will fall to pieces. - -The glass imitations of pearls, which have become very common in recent -years, may, apart from their inferior iridescence, be detected by -their greater hardness, or by the apparent doubling of, say, a spot of -ink placed on the surface, owing to reflection from the inner surface -of the glass shell. They are made of small hollow spheres formed by -blowing. Next to the glass comes a lining of parchment size, and next -the under lining, which is the most important part of the imitation, -consisting of a preparation of fish scales called _Essence d’Orient_, -When the lining is dry, the globe is filled with hot wax to impart the -necessary solidity. In cheap imitations the glass balls are not lined -at all, but merely heated with hydrochloric acid to give an iridescence -to the surface; sometimes they are coated with wax, which can be -scraped off with a knife. - - - - - PART II—SECTION A - - PRECIOUS STONES - - - - - CHAPTER XVI - - DIAMOND - - -Diamond has held pride of place as chief of precious stones ever -since the discovery of the form of cutting known as the ‘brilliant’ -revealed to full perfection its amazing qualities; and justly so, -since it combines in itself extreme hardness, high refraction, large -colour-dispersion, and brilliant lustre. A rough diamond, especially -from river gravels, has often a peculiar greasy appearance, and is -no more attractive to the eye than a piece of washing-soda. It is -therefore easy to understand why the Persians in the thirteenth century -placed the pearl, ruby, emerald, and even peridot before it, and -writers in the Middle Ages frequently esteemed it below emerald and -ruby. The Indian lapidaries, who were the first to realize that diamond -could be ground with its own powder, discovered what a wonderful -difference the removal of the skin makes in the appearance of a stone. -They, however, made no attempt to shape a stone, but merely polished -the natural facets, and only added numerous small facets when they -wished to conceal flaws or other imperfections; indeed, the famous -traveller, Tavernier, from whom most of our knowledge of early mining -in India is obtained, invariably found that a stone covered with many -facets was badly flawed. The full radiant beauty of a diamond comes to -light only when it is cut in brilliant form. - -Of all precious stones diamond has the simplest composition; it is -merely crystallized carbon, another form of which is the humble and -useful graphite, commonly known as ‘black-lead.’ Surely nature has -surpassed all her marvellous efforts in producing from the same element -substances with such divergent characters as the hard, brilliant, and -transparent diamond and the soft, dull, and opaque graphite. It is, -however, impossible to draw any sharp dividing line between the two; -soft diamond passes insensibly into hard graphite, and vice versa. -Boart, or bort, as it is sometimes written, is composed of minute -crystals of diamond arranged haphazardly; it possesses no cleavage, -its hardness is greater than that of the crystals, and its colour is -greyish to blackish. Carbon, carbonado, or black diamond, which is -composed of still more minute crystals, is black and opaque, and is -perceptibly harder than the crystals. It passes into graphite, which -varies in hardness, and may have any density between 2·O and 3·O. -Jewellers apply the term boart to crystals or fragments which are of no -service as gems; such pieces are crushed to powder and used for cutting -and polishing purposes. - -Diamonds, when absolutely limpid and free from flaws, are said to be -of the ‘first water,’ and are most prized when devoid of any tinge of -colour except perhaps bluish (Plate I, Fig. 1). Stones with a slight -tinge of yellow are termed ‘off-coloured,’ and are far less valuable. -Those of a canary-yellow colour (Plate I, Fig. 3), however, belong to a -different category, and have a decided attractiveness. Greenish stones -also are common, though it is rare to come across one with a really -good shade of that colour. Brown stones, especially in South Africa, -are not uncommon. Pink stones are less common, and ruby-red and blue -stones are rare. Those of the last-named colour have usually what is -known as a ‘steely’ shade, _i.e._ they are tinged with green; stones of -a sapphire blue are very seldom met with, and such command high prices. - -[Illustration: FIGS. 57—59.—Diamond Crystals.] - -Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra -with brilliant, smooth faces, and occasionally in cubes with rough -pitted faces; sometimes three or six faces take the place of each -octahedron face, and the stone is almost spherical in shape. The -surfaces of the crystals are often marked with equilateral triangles, -which are supposed to represent the effects of incipient combustion. -Twinned crystals, in which the two individuals may be connected by a -single plane or may be interpenetrating, a star shape often resulting -in the latter case, are common; sometimes, if of the octahedron type, -they are beautifully symmetrical. The rounded crystals are frequently -covered with a peculiar gum-like skin which is somewhat less hard than -the crystal itself. A large South African stone, weighing 27 grams (130 -carats) and octahedral in shape, which was the gift of John Ruskin, -and named by him the ‘Colenso’ after the first bishop of Natal, is -exhibited in the British Museum (Natural History); its appearance is, -however, marred by its distinctly ‘off-coloured’ tint. - -The refraction of diamond is single, but local double refraction is -common, indicating a state of strain which can often be traced to an -included drop of liquid carbonic acid; so great is the strain that many -a fine stone has burst to fragments on being removed from the ground -in which it has lain. The refractive index for the yellow light of a -sodium flame is 2·4175, and the slight variation from this mean value -that has been observed, amounting only to 0·0001, testifies to the -purity of the composition. The colour-dispersion is large, being as -much as 0·044, in which respect it surpasses all colourless stones, -but is exceeded by sphene and the green garnet from the Urals (cf. p. -217). The lustre of diamond, when polished, is so characteristic as to -be termed adamantine, and is due to the combination of high refraction -and extreme hardness. Diamond is translucent to the X (Röntgen) rays; -it phosphoresces under the action of radium, and of a high-tension -electric current when placed in a vacuum tube, and sometimes even when -exposed to strong sunlight. Some diamonds fluoresce in sunlight, -turning milky, and a few even emit light when rubbed. Crookes found -that a diamond buried in radium bromide for a year had acquired a -lovely blue tint, which was not affected even by heating to redness. -The specific gravity is likewise constant, being 3·521, with a possible -variation from that mean value of 0·005; but a greater range, as might -be expected, is found in the impure boart. - -Diamond is by far the hardest substance in nature, being marked 10 in -Mohs’s scale of hardness, but it varies in itself; stones from Borneo -and New South Wales are so perceptibly harder than those usually in -the lapidaries’ hands, that they can be cut only with their own and -not ordinary diamond powder, and some difficulty was experienced in -cutting them when they first came into the market. It is interesting -to note that the metal tantalum, the isolation of which in commercial -amount constituted one of the triumphs of chemistry of recent years, -has about the same hardness as diamond. Despite its extreme hardness -diamond readily cleaves under a heavy blow in planes parallel to the -faces of the regular octahedron, a property utilized for shaping the -stone previous to cutting it. The fallacious, but not unnatural, -idea was prevalent up to quite modern times that a diamond would, -even if placed on an anvil, resist a blow from a hammer: who knows -how many fine stones have succumbed to this illusory test? The fact -that diamond could be split was known to Indian lapidaries at the -time of Tavernier’s visit, and it would appear from De Boodt that in -the sixteenth century the cleavability of diamond was not unknown in -Europe, but it was not credited at the time and was soon forgotten. -Early last century Wollaston, a famous chemist and mineralogist, -rediscovered the property, and, so it is said, used his knowledge to -some profit by purchasing large stones, which because of their awkward -shape or the presence of flaws in the interior were rejected by the -lapidaries, and selling them back again after cleaving them to suitable -forms. - -It has already been remarked (p. 79) that the interval in hardness -between diamond and corundum, which comes next to it in Mohs’s scale, -is enormously greater than that between corundum and the softest of -minerals. Diamond can therefore be cut only with the aid of its own -powder, and the cutting of diamond is therefore differentiated from -that of other stones, the precious-stone trade being to a large extent -divided into two distinct groups, namely, dealers in diamonds, and -dealers in all other gem-stones. - -The name of the species is derived from the popular form, _adiamentem_, -of the Latin _adamantem_, itself the alliterative form of the Greek -ἀδάμας, meaning the unconquerable, in allusion not merely to the -great hardness but also to the mistaken idea already mentioned. Boart -probably comes from the Old-French _bord_ or _bort_, bastard. - -At the present day diamonds are usually cut as brilliants, though -the contour of the girdle may be circular, oval, or drop-shaped to -suit the particular purpose for which the stone is required, or to -keep the weight as great as possible. Small stones for bordering a -large coloured stone may also be cut as roses or points. A perfect -brilliant has 58 facets, but small stones may have not more than 44, -and exceptionally large stones may with advantage have many more; for -instance, on the largest stone cut from the Cullinan diamond there are -no fewer than 74 facets. - -The description of the properties of diamond would not be complete -without a reference to the other valuable, if utilitarian, purposes -to which it is put. Without its aid much of modern engineering work -and mining operations would be impossible except at the cost of almost -prohibitive expenditure of time and money. - -Boring through solid rock has been greatly facilitated by the use of -the diamond drill. For this purpose carbonado or black diamond is more -serviceable than single crystals, and the price of the former has -consequently advanced from a nominal figure up to £3 to £12 a carat. -The actual working part of the drill consists of a cast-steel ring. -The crown of it has a number of small depressions at regular intervals -into which the carbonados are embedded. On revolution of the drill an -annular ring is cut, leaving a solid core which can be drawn to the -surface. For cooling the drill and for washing away the detritus water -is pumped through to the working face. The duration of the carbonados -depends on the nature of the rock and the skill of the operator. The -most troublesome rock is a sandstone or one with sharp differences in -hardness, because the carbonados are liable to be torn out of their -setting. An experienced operator can tell by the feel of the drill the -nature of the rock at the working face, and by varying the pressure can -mitigate the risk of damage to the drill. - -The tenacity of diamond renders it most suitable for wire-drawing. The -tungsten filaments used in many of the latest forms of incandescent -electric lamps are prepared in this manner. - -Diamond powder is used for cutting and turning the hardened steel -employed in modern armaments and for other more peaceful purposes. - -Although nearly all the gem-stones scratch glass, diamond alone can be -satisfactorily employed to cut it along a definite edge. Any flake at -random will not be suitable, because it will tear the glass and form a -jagged edge. The best results are given by the junction of two edges -which do not meet in too obtuse an angle; two edges of the rhombic -dodecahedron meet the requirements admirably. The stones used by the -glaziers are minute in size, being not much larger than a pin’s head, -and thirty of them on an average go to the carat. They are set in -copper or brass. Some little skill is needed to obtain the best results. - -The value of a diamond has always been determined largely by the size -of the stone, the old rule being that the rate per carat should be -multiplied by the square of the weight in carats; thus, if the rate be -£10, the cost of a two-carat stone is four times this sum, or £40, of -a three-carat stone £90, and so on. For a century, from 1750 to 1850, -the rate remained almost constant at £4 for rough, £6 for rose-cut, -and £8 for brilliant-cut diamonds. Since the latter date, owing to -the increase in the supply of gold, the growth of the spending power -of the world, and the gradual falling off in the productiveness of -the Brazilian fields, the rate steadily increased about 10 per cent. -each year, until in 1865 the rate for brilliants was £18. The rise was -checked by the discovery of the South African mines; moreover. since -comparatively large stones are plentiful in these mines, the rule -of the increase in the price of a stone by the square of its weight -no longer holds. The rate for the most perfect stones still remains -high, because such are not so common in the South African mines. The -classification[7] adopted by the syndicate of London diamond merchants -who place upon the market the output of the De Beers group of mines is -as follows:—(_a_) Blue-white, (_b_) white, (_c_) silvery Cape, (_d_) -fine Cape, (_e_) Cape, (_f_) fine bywater, (_g_) bywater, (_h_) fine -light brown, (_i_) light brown, (_j_) brown, (_k_) dark brown. Bywaters -or byes are stones tinged with yellow. - -The rate per carat for cut stones in the blue-white and the bywater -groups is:— - - BLUE-WHITE. BYWATER. - 5-carat stone £40-60 £20-25 - 1 „ 30-40 10-15 - ½ „ 20-25 8-12 - ¼ „ 15-18 6-10 - Mêlée 12-15 5-8 - -Mêlée are stones smaller than a quarter of a carat. It will be noticed -that the prices depart largely from the old rule; thus taking the rate -for a carat blue-white stone, the price of a five-carat stone should -be from £150-200 a carat, and for a quarter-carat stone only £7, 10s. -to £10 a carat. There happens to be at the time of writing very little -demand for five-carat stones. Of course, the prices given are subject -to constant fluctuation depending upon the supply and demand, and the -whims of fashion. - - - - - CHAPTER XVII - - OCCURRENCE OF DIAMOND - - -The whole of the diamonds known in ancient times were obtained from -the so-called Golconda mines in India. Golconda itself, now a deserted -fortress near Hyderabad, was merely the mart where the diamonds were -bought and sold. The diamond-bearing district actually spread over a -wide area on the eastern side of the Deccan, extending from the Pinner -River in the Madras Presidency northwards to the Rivers Son and Khan, -tributaries of the Ganges, in Bundelkhand. The richest mines, where -the large historical stones were found, are in the south, mostly -near the Kistna River. The diamonds were discovered in sandstone, -or conglomerate, or the sands and gravels of river-beds. The mines -were visited in the middle of the seventeenth century by the French -traveller and jeweller, Tavernier, when travelling on a commission -for Louis XIV, and he afterwards published a careful description of -them and of the method of working them. The mines seem to have been -exhausted in the seventeenth century; at any rate, the prospecting, -which has been spasmodically carried on during the last two centuries, -has proved almost abortive. With the exception of the Koh-i-nor, all -the large Indian diamonds were probably discovered not long before -Tavernier’s visit. The diamonds known to Pliny, and in his time, were -quite small, and it is doubtful if any stones of considerable size came -to light before A.D. 1000. - -India enjoyed the monopoly of supplying the world’s demand for diamonds -up to the discovery, in 1725, of the precious stone in Brazil. Small -stones were detected by the miners in the gold washings at Tejuco, -about eighty miles (129 km.) from Rio de Janeiro, in the Serro do Frio -district of the State of Minas Geraes. The discovery naturally caused -great excitement. So many diamonds were found that in 1727 something -like a slump took place in their value. In order to keep up prices, the -Dutch merchants, who mainly controlled the Indian output, asserted that -the diamonds had not been found in Brazil at all, but were inferior -Indian stones shipped to Brazil from Goa. The tables were neatly turned -when diamonds were actually shipped from Brazil to Goa, and exported -thence to Europe as Indian stones. This course and the continuous -development of the diamond district in Brazil rendered it impossible -to hoodwink the world indefinitely. The drop in prices was, however, -stayed by the action of the Portuguese government, who exacted such -heavy duties and imposed such onerous conditions that finally no one -would undertake to work the mines. Accordingly, in 1772 diamond-mining -was declared a royal monopoly in Brazil, and such it remained until -the severance of Brazil from Portugal in 1834, when private mining was -permitted by the new government subject to the payment of reasonable -royalties. The industry was enormously stimulated by the discovery, in -1844, of the remarkably rich fields in the State of Bahia, especially -at Serra da Cincorá, where carbonado, or black diamond, first came to -light, but after a few years, owing to the difficulties of supplying -labour, the unhealthiness of the climate, and the high cost of living, -the yield fell off and gradually declined, until the importance of the -fields was finally eclipsed by the rise of the South African mines. -The Brazilian mines have proved very productive, but chiefly in small -diamonds, stones above a carat in weight being few in comparison. The -largest stone, to which the name, the Star of the South, was applied, -weighed in the rough 254½ carats; it was discovered at the Bagagem -mines in 1853. The quality of the diamonds is good, many of them having -the highly-prized bluish-white colour. The principal diamond-bearing -districts of Brazil centre at Diamantina, as Tejuco was re-named -after the discovery of diamonds, Grão Magor, and Bagagem in the State -of Minas Geraes, at Diamantina in the State of Bahia, and at Goyãz -and Matto Grosso in the States of the same names. The diamonds occur -chiefly in _cascalho_, a gravel, containing large masses of quartz -and small particles of gold, which is supposed to be derived from a -quartzose variety of micaceous slate known as itacolumite. The mines -are now to some extent being worked by systematic dredging of the -river-beds. - -Early in 1867 the children of a Boer farmer, Daniel Jacobs, who dwelt -near Hopetown on the banks of the Orange River, picked up in the course -of play near the river a white pebble, which was destined not only to -mark the commencement of a new epoch in the record of diamond mines, -but to change the whole course of the history of South Africa. This -pebble attracted the attention of a neighbour, Schalk van Niekerk, who -suspected that it might be of some value, and offered to buy it. Mrs. -Jacobs, however, gave it him, laughingly scouting the idea of accepting -money for a mere pebble. Van Niekerk showed it to a travelling trader, -by name John O’Reilly, who undertook to obtain what he could for it on -condition that they shared the proceeds. Every one he met laughed to -scorn the idea that the stone had any value, and it was once thrown -away and only recovered after some search in a yard, but at length he -showed it to Lorenzo Boyes, the Acting Civil Commissioner at Colesberg, -who, from its extreme hardness, thought it might be diamond and sent -it to the mineralogist, W. Guybon Atherston, of Grahamstown, for -determination. So uncertain was Boyes of its value that he did not even -seal up the envelope containing it, much less register the package. -Atherston found immediately that the long-scorned pebble was really -a fine diamond, weighing 21-3/16 carats, and with O’Reilly’s consent -he submitted it to Sir Philip Wodehouse, Governor at the Cape. The -latter purchased it at once for £500, and dispatched it to be shown -at the Paris Exhibition of that year. It did not, however, attract -much attention; chimerical tales of diamond finds in remote parts of -the world are not unknown. Indeed, for some time only a few small -stones were picked up beside the Orange River, and no one believed in -the existence of any extensive diamond deposit. However, all doubt as -to the advisibility of prospecting the district was settled by the -discovery of the superb diamond, afterwards known as the ‘Star of -South Africa,’ which was picked up in March 1869 by a shepherd boy -on the Zendfontein farm near the Orange River. Van Niekerk, on the -alert for news of further discoveries, at once hurried to the spot and -purchased the stone from the boy for five hundred sheep, ten oxen, and -a horse, which seemed to the boy untold wealth, but was not a tithe of -the £11,200 which Lilienfeld Bros., of Hopetown, gave Van Niekerk. - -[Illustration: _PLATE XV_ - -KIMBERLEY MINE, 1871] - -[Illustration: _PLATE XVI_ - -KIMBERLEY MINE, 1872] - -This remarkable discovery attracted immediate attention to the -potentialities of a country which produced diamonds of such a size, and -prospectors began to swarm into the district, gradually spreading up -the Vaal River. For some little time not much success was experienced, -but at length, early in 1870, a rich find was made at Klipdrift, -now known as Barkly West, which was on the banks of the Vaal River -immediately opposite the Mission camp at Pniel. The number of miners -steadily increased until the population on the two sides of the river -included altogether some four or five thousand people, and there was -every appearance of stability in the existing order of things. But a -vast change came over the scene upon the discovery of still richer -mines lying to the south-east and some distance from the river. The -ground was actually situated on the route traversed by parties hurrying -to the Vaal River, but no one dreamed of the wealth that lay under -their feet. The first discovery was made in August 1870 at the farm -Jagersfontein, near Fauresmith in Orange River Colony, by De Klerk, -the intelligent overseer, who noticed in the dry bed of a stream a -number of garnets, and, knowing that they often accompanied diamond, -had the curiosity to investigate the point. He was immediately -rewarded by finding a fine diamond weighing 50 carats. In the following -month diamonds were discovered about twenty miles from Klipdrift -at Dutoitspan on the Dorstfontein farm, and a little later also on -the contiguous farm of Bultfontein; a diamond was actually found in -the mortar used in the homestead of the latter farm. Early in May -1871 diamonds were found about two miles away on De Beers’ farm, -Vooruitzigt, and two months later, in July, a far richer find was made -on the same farm at a spot which was first named Colesberg Kopje, the -initial band of prospectors having come from the town of that name -near the Orange River, but was subsequently known as Kimberley after -the Secretary of State for the Colonies at that time. Soon a large -and prosperous town sprang up close to the mines; it rapidly grew -in size and importance, and to this day remains the centre of the -diamond-mining industry. Subsequent prospecting proved almost blank -until the discovery of the Premier or Wesselton mine on Wesselton -farm, about four miles from Kimberley, in September 1890; it received -the former name after Rhodes, who was Premier of Cape Colony at that -date. No further discovery of any importance was made until, in 1902, -diamonds were found about twenty miles north-west-north of Pretoria in -the Transvaal, at the new Premier mine, now famed as the producer of -the gigantic Cullinan diamond. - -[Illustration: _PLATE XVII_ - -KIMBERLEY MINE, 1874] - -[Illustration: _PLATE XVIII_ - -KIMBERLEY MINE, 1881] - -The Kimberley mines were at first known as the ‘dry diggings’ on -account of their arid surroundings in contradistinction to the ‘river -diggings’ by the Vaal. The dearth of water was at first one of the -great difficulties in the way of working the former mines, although -subsequently the accumulation of underground water at lower levels -proved a great obstacle to the working of the mines. The ‘river -diggings’ were of a type similar to that met with in India and Brazil, -the diamonds occurring in a gravelly deposit of limited thickness -beneath which was barren rock, but the Kimberley mines presented a -phenomenon hitherto without precedent in the whole history of diamond -mining. The diamonds were found in a loose surface deposit, which was -easily worked, and for some time the prospectors thought that the -underlying limestone corresponded to the bedrock of the river gravel, -until at length one more curious than his fellows investigated the -yellowish ground underneath, and found to his surprise that it was even -richer than the surface layer. Immediately a rush was made back to the -deserted claims, and the mines were busier than ever. This ‘yellow -ground,’ as it is popularly called, was much decomposed and easy, -therefore, to work and sift. About fifty to sixty feet (15-18 m.) below -the surface, however, it passed into a far harder rock, which from its -colour is known as the ‘blue ground’; this also, to the unexpected -pleasure of the miners, turned out to contain diamonds. Difficulties -arose as each claim, 30 by 30 Dutch feet (about 31 English feet or -9·45 metres square) in area, was worked downwards. In the Kimberley -mine (Plate XVI) access to the various claims was secured by retaining -parallel strips, 15 feet wide, each claim being, therefore, reduced -in width to 22½ feet, to form roadways running from side to side of -the mine in one direction. These, however, soon gave way, not only -because of the falling of the earth composing them, but because they -were undermined and undercut by the owners of the adjacent claims. -By the end of 1872 the last roadway had disappeared, and the mine -presented the appearance of a vast pit. In order to obtain access to -the claims without intruding on those lying between, and to provide for -the hauling of the loads of earth to the surface, an ingenious system -of wire cables in three tiers (Plate XVII) was erected, the lowest -tier being connected to the outermost claims, the second to claims -farther from the edge, and the highest to claims in the centre of the -pit. The mine at that date presented a most remarkable spectacle, -resembling an enormous radiating cobweb, which had a weird charm by -night as the moonlight softly illuminated it, and by day, owing to -the perpetual ring of the flanged wheels of the trucks on the running -wires, twanged like some gigantic æolian harp. This system fulfilled -its purpose admirably until, with increasing depth of the workings, -other serious difficulties arose. Deprived of the support of the hard -blue ground, the walls of the mine tended to collapse, and additional -trouble was caused by the underground water that percolated into the -mine. By the end of 1883 the floor of the Kimberley mine was almost -entirely covered by falls of ‘reef’ (Plate XVIII), as the surrounding -rocks are termed, the depth then being about 400 feet (122 m.). In the -De Beers mine, in spite of the precaution taken to prevent falls of -reef by cutting the walls of the mine back in terraces, falls occurred -continuously in 1884, and by 1887, at a depth of 350 feet (107 m.), -all attempts at open working had to be abandoned. In the Dutoitspan -mine buttresses of blue ground were left, which held back the reef for -some years, but ultimately the mine became unsafe, and in March 1886 a -disastrous fall took place, in which eighteen miners—eight white men -and ten Kafirs—lost their lives. The Bultfontein mine was worked to the -great depth of 500 feet (152 m.), but falls occurred in 1889 and put -an end to open working. In all cases, therefore, the ultimate end was -the same: the floor of the mine became covered with a mass of worthless -reef, which rendered mining from above ground dangerous, and, indeed, -impossible except at prohibitive cost. It was then clearly necessary -to effect access to the diamond-bearing ground by means of shafts sunk -at a sufficient distance from the mine to remove any fear of falls of -reef. For such schemes co-operative working was absolutely essential. -Plate XIX illustrates the desolate character of the Kimberley mine -above ground and the vastness of the yawning pit, which is over 1000 -feet (300 m.) in depth. - -[Illustration: _PLATE XIX_ - -KIMBERLEY MINE AT THE PRESENT DAY] - -[Illustration: _PLATE XX_ - -WESSELTON (_open_) MINE] - -A certain amount of linking up of claims had already taken place, but, -although many men must have seen that the complete amalgamation of the -interests in each mine was imperative, two men alone had the capacity -to bring their ideas to fruition. C. J. Rhodes was the principal agent -in the formation in April 1880 of the De Beers Mining Company, which -rapidly absorbed the remaining claims in the mine, and was re-formed -in 1887 as the De Beers Consolidated Mining Company. Meantime, Barnett -Isaacs, better known by the cognomen Barnato, which had been adopted by -his - -brother Henry when engaged in earning his livelihood in the diamond -fields as an entertainer, had secured the major interests in the -Kimberley mine. Rhodes saw that, for effective working of the two -mines by any system of underground working, they must be under one -management, but to all suggestions of amalgamation Barnato remained -deaf, and at last Rhodes determined to secure control of the Kimberley -mine at all costs. The story of the titanic struggle between these -two men forms one of the epics of finance. Eventually, when shares -in the Kimberley mine had been boomed to an extraordinary height, -and the price of diamonds had fallen as low as 18s. a carat, Barnato -gave way, and in July 1889 the Kimberley mine was absorbed by the De -Beers Company on payment of the enormous sum of £5,338,650. Shortly -afterwards they undertook the working of the Dutoitspan and the -Bultfontein mines, and in January 1896 they acquired the Premier or -Wesselton mine. The interests in the Jagersfontein mine were in 1888 -united in the New Jagersfontein Mining and Exploration Company, and -the mine is now worked also by the De Beers Company. Thus, until the -development of the new Premier mine in the Transvaal, the De Beers -Company practically controlled the diamond market. The development -of this last mine was begun so recently, and its size is so vast—the -longest diameter being half a mile—that open-cut working is likely to -continue for some years. - -[Illustration: _PLATE XXI_ - -LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT OVER] - -[Illustration: _PLATE XXII_ - -WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND] - -Though varying slightly in details, the methods of working the mines -are identical in principle. From the steeply inclined shaft horizontal -galleries are run diagonally right across the mine, the vertical -interval between successive galleries being 40 feet. From each -gallery side galleries are run at right angles to it and parallel to -the working face. The blue ground is worked systematically backwards -from the working face. The mass is stoped, _i.e._ drilled and broken -from the bottom upwards, until only a thin roof is left. As soon as the -section is worked out and the material removed, the roof is allowed -to fall in, and work is begun on the next section of the same level; -at the same time the first section on the level next below is opened -out. Thus work is simultaneously carried on in several levels, and a -vertical plane would intersect the working faces in a straight line -obliquely inclined to the vertical direction (Fig. 60). When freshly -mined, the blue ground is hard and compact, but it soon disintegrates -under atmospheric influence. Indeed, the yellow ground itself was -merely decomposed blue ground. No immediate attempt is made, therefore, -to retrieve the precious stones. The blue ground is spread on to -the ‘floors’ (Plate XXI), _i.e._ spaces of open veldt which have -been cleared of bushes and inequalities, to the depth of a couple of -feet, and remains there for periods ranging from six months to two -years, depending on the quality of the blue ground and the amount of -rainfall. To hasten the disintegration the blue ground is frequently -ploughed over and occasionally watered, a remarkable introduction of -agricultural methods into mining operations. No elaborate patrolling -or guarding is required, because the diamonds are so sparsely, though -regularly, scattered through the mass that even of the actual workers -in the mines but few have ever seen a stone in the blue ground. When -sufficiently broken up, it is carted to the washing and concentrating -machines, by means of which the diamonds and the heavier constituents -are separated from the lighter material. - -[Illustration: FIG. 60.—Vertical Section of Diamond Pipe, showing -Tunnels and Stopes.] - -Formerly the diamonds were picked out from the concentrates by means -of the keen eyes of skilled natives; but the process has been vastly -simplified and the risk of theft entirely eliminated by the remarkable -discovery made in 1897 by F. Kirsten, of the De Beers Company, that -of all the heavy constituents of the blue ground diamond alone, with -the exception of an occasional corundum and zircon, which are easily -sorted out afterwards, adheres to grease more readily than to water. -In this ingenious machine, the ‘jigger’ or ‘greaser’ (Plate XXIII) as -it is commonly termed, the concentrates are washed over a series of -galvanized-iron trays, which are covered with a thick coat of grease. -The trays are slightly inclined downwards, and are kept by machinery -in constant sideways motion backwards and forwards. So accurate is the -working of this device that few diamonds succeed in getting beyond -the first tray, and none progress as far as the third, which is added -as an additional precaution. The whole apparatus is securely covered -in so that there is no risk of theft during the operation. The trays -are periodically removed, and the grease is scraped off and boiled to -release the diamonds, the grease itself being used over again on the -trays. This is the first time in the whole course of extraction from -the mines that the diamonds are actually handled. The stones are now -passed on to the sorters, who separate them into parcels according to -their size, shape, and quality. - -[Illustration: _PLATE XXIII_ - -DIAMOND-SORTING MACHINES] - -[Illustration: _PLATE XXIV_ - -KAFIRS PICKING OUT DIAMONDS] - -The classification at the mines is first into groups by the shape: -(1) close goods, (2) spotted stones, (3) rejection cleavage, (4) -fine cleavage, (5) light brown cleavage, (6) ordinary and rejection -cleavage, (7) flats, (8) macles, (9) rubbish, (10) boart. Close goods -are whole crystals which contain no flaws and can be cut into single -stones. Spotted stones, as their name suggests, contain spots which -necessitate removal, and cleavage includes stones which are so full of -flaws that they have to be cleaved or split into two or more stones. -Flats are distorted octahedra, and macles are twinned octahedra. -Rubbish is material which can be utilized only for grinding purposes, -and boart consists of round dark stones which are invaluable for -rock-drills. These groups are afterwards graded into the following -subdivisions, depending on increasing depth of yellowish tint: (_a_) -blue-white, (_b_) first Cape, (_c_) second Cape, (_d_) first bye, (_e_) -second bye, (_f_) off-colour, (_g_) light yellow, (_h_) yellow. It is, -however, only the first group that is so minutely subdivided. After -being purchased, the parcels are split up again somewhat differently -for the London market (cf. p. 136), and the dealers re-arrange the -stones according to the purpose for which they are required. Formerly -a syndicate of London merchants took the whole of the produce of the -Kimberley mines at a previously arranged price per carat, but at -the present time the diamonds are sold by certain London firms on -commission. - -The products of each mine show differences in either form or colour -which enable an expert readily to recognize their origin. The old -diggings by the Vaal River yielded finer and more colourless stones -than those found in the dry diggings and the mines underlying them. The -South African diamonds, taken as a whole, are always slightly yellowish -or ‘off-coloured’; the mines are, indeed, remarkable for the number of -fine and large, canary-yellow and brown, stones produced. The Kimberley -mine yields a fair percentage of white, and a large number of twinned -and yellow stones. The yield of the De Beers mine comprises mostly -tinted stones—yellow and brown, occasionally silver capes, and very -seldom stones free from colour. The Dutoitspan mine is noted for its -harvest of large yellow diamonds; it also produces fine white cleavage -and small white octahedra. The stones found in the Bultfontein mine are -small and spotted, but, on the other hand, the yield has been unusually -regular. The Premier or Wesselton mine yields a large proportion -of flawless octahedra, but, above all, a large number of beautiful -deep-orange diamonds. Of all the South African mines the Jagersfontein -in the Orange River Colony alone supplies stones of the highly-prized -blue-white colour and steely lustre characteristic of the old Indian -stones. The new Premier mine in the Transvaal is prolific, but mostly -in off-coloured and low-grade stones, the Cullinan diamond being a -remarkable exception. - -To illustrate the amazing productiveness of the South African mines, -it may be mentioned that, according to Gardner F. Williams, the -Kimberley group of mines in sixteen years yielded 36 million carats -of diamonds, and the annual output of the Jagersfontein mine averages -about a quarter of a million carats, whereas the total output of the -Brazil mines, for the whole of the long period during which they have -been worked, barely exceeds 13 million carats. The average yield of the -South African mines, however, perceptibly diminishes as the depth of -the mines increases. - -The most interesting point connected with the South African diamond -mines, viewed from the scientific standpoint, is the light that they -have thrown on the question of the origin of the diamond, which -previously was an incomprehensible and apparently insoluble problem. -In the older mines, just as at the river diggings by the Vaal, the -stones are found in a gravelly deposit that has resulted from the -disintegration of the rocks through which the adjacent river has -passed, and it is clear that the diamond cannot have been formed _in -situ_ here; it had been suspected, and now there is no doubt, that the -itacolumite rock of Brazil has consolidated round the diamonds which -are scattered through it, and that it cannot be the parent rock. The -occurrence at Kimberley is very different. These mines are funnels -which go downwards to unknown depths; they are more or less oval in -section, becoming narrower with increasing depth, and are evidently the -result of some eruptive agency. The Kimberley mine has been worked to -a depth of nearly 4000 feet (1200 m.), and no signs of a termination -have as yet appeared. The blue ground which fills these ‘pipes,’ as -they are termed, must have been forced up from below, since it is -sharply differentiated from the surrounding country rocks. This blue -ground is a brecciated peridotite of peculiar constitution, to which -the well-known petrologist, Carvil Lewis, who made a careful study -of it, gave the name kimberlite. The blue colour testifies to its -richness in iron, and it is to the oxidation of the iron constituent, -that the change of colour to yellow in the upper levels is due. Owing -to the shafts that have been sunk for working the mines, the nature -of the surrounding rocks is known to some depth. Immediately below -the surface is a decomposed ferriferous basalt, about 20 to 90 feet -(6-27 m.) thick, next a black slaty shale, 200 to 250 feet (60-75 m.) -thick, then 10 feet (3 m.) of conglomerate, next 400 feet (120 m.) -of olivine diabase, then quartzite, about 400 feet (120 m.) thick, -and lastly a quartz porphyry, which has not yet been penetrated. The -strata run nearly horizontal, and there are no signs of upward bending -at the pipes. The whole of the country, including the mines, was -covered with a red sandy soil, and there was nothing to indicate the -wealth that lay underneath. The action of water had in process of time -removed all signs of eruptive activity. The principal minerals which -are associated with diamond in the blue ground are magnetite, ilmenite, -chromic pyrope, which is put on the market as a gem under the misnomer -‘Cape-ruby,’ ferriferous enstatite, which also is sometimes cut, -olivine more or less decomposed, zircon, kyanite, and mica. - -The evidence produced by an examination of the blue ground and the -walls of the pipes proves that the pipes cannot have been volcanoes -such as Vesuvius. There is no indication whatever of the action of any -excessive temperature, while, on the other hand, there is every sign of -the operation of enormous pressure; the diamonds often contain liquid -drops of carbonic acid. Crookes puts forward the plausible theory that -steam has been the primary agency in propelling the diamond and its -associates up into the channel through which it has carved its way to -freedom, and holds that molten iron has been the solvent for carbon -which has crystallized out as diamond under the enormous pressures -obtaining in remote depths of the earth’s crust. It is pertinent to -note that, by dissolving carbon in molten iron, the eminent chemist, -Moissan, was enabled to manufacture tiny diamond crystals. Water -trickling down from above would be immediately converted into steam at -very high pressure on coming into contact with the molten iron, and, in -its efforts to escape, the steam would drive the iron and its precious -contents, together with the adjacent rocks, upwards to the surface. The -ferriferous nature of the blue ground and the yellow tinge so common -to the diamonds lend confirmation to this theory. The process by which -the carbon was extracted from shales or other carboniferous rocks and -dissolved in iron still awaits elucidation. - -Diamonds were found in New South Wales as long ago as 1851 on Turon -River and at Reedy Creek, near Bathurst, about ninety miles (145 km.) -from Sydney, but the find was of little commercial importance. A more -extensive deposit came to light in 1867 farther north at Mudgee. In -1872 diamonds were discovered in the extreme north of the State, at -Bingara near the Queensland border. Another discovery was made in 1884 -at Tingha, and still more recently in the tin gravels of Inverell in -the same region. In their freedom from colour and absence of twinning -the New South Wales diamonds resemble the Brazilian stones. The average -size is small, running about five to the carat when cut; the largest -found weighed nearly 6 carats when cut. They are remarkable for their -excessive hardness; they can be cut only with their own dust, ordinary -diamond dust making no impression. - -The Borneo diamonds are likewise distinguished by their exceptional -hardness. They mostly occur by the river Landak, near Pontianak on the -west coast of the island. They are found in a layer of rather coarse -gravel, variable, but rarely exceeding a yard (1 m.), in depth, and are -associated with corundum and rutile, together with the precious metals -gold and platinum. Indeed, it is no uncommon sight to see natives -wearing waistcoats ornamented with gold buttons, in each of which a -diamond is set. The diamonds are well crystallized and generally of -pure water; yellowish and canary-yellow stones are also common, but -rose-red, bluish, smoky, and black stones are rare. They seldom exceed -a carat in weight; but stones of 10 carats in weight are found, and -occasionally they attain to 20 carats. In 1850 a diamond weighing 77 -carats was discovered. The Rajah of Mattan is said to possess one of -the purest water weighing as much as 367 carats, but no one qualified -to pronounce an opinion regarding its genuineness has ever seen it. - -In Rhodesia small diamonds have been found in gravel beds resting on -decomposed granite near the Somabula forest, about 12 miles (19 km.) -west of Gwelo, in association with chrysoberyl in abundance, blue -topaz, kyanite, ruby, sapphire, tourmaline, and garnet. - -The occurrence of diamond in German South-West Africa is very peculiar. -Large numbers of small stones are found close to the shore near -Luderitz Bay in a gravelly surface layer, which is nowhere more than a -foot in depth. They are picked by hand by natives and washed in sieves. -In shape they are generally six-faced octahedra or twinned octahedra, -simple octahedra being rare, and in size they run about four or five -to the carat, the largest stone as yet found being only 2 carats in -weight. Their colour is usually yellowish. - -Several isolated finds of diamonds have been reported in California -and other parts of the United States, but none have proved of any -importance. The largest stone found weighed 23¾ carats uncut; it was -discovered at Manchester in Virginia. - - - - - CHAPTER XVIII - - HISTORICAL DIAMONDS - - -The number of diamonds which exceed a hundred carats in weight when -cut is very limited. Their extreme costliness renders them something -more than mere ornaments; in a condensed and portable form they -represent great wealth and all the potentiality for good or ill thereby -entailed, and have played no small, if sinister, rôle in the moulding -of history. In bygone days when despotic government was universal, the -possession of a splendid jewel in weak hands but too often precipitated -the aggression of a greedy and powerful neighbour, and plunged whole -countries into the horrors of a ruthless and bloody war. In more -civilized days a great diamond has often been pledged as security for -money to replenish an empty treasury in times of stress. The ambitions -of Napoleon might have received a set-back but for the funds raised -on the security of the famous Pitt diamond. The history of such -stones—often one long romance—is full of interest, but space will not -permit of more than a brief sketch here. - -If we except the colossal Cullinan stone, the mines of Brazil and South -Africa cannot compare with the old mines of India as the birthplace of -large and perfect diamonds of world-wide fame. - - - (1) KOH-I-NOR - -[Illustration: FIG. 61.—Koh-i-nor (top view).] - -[Illustration: FIG. 62.—Koh-i-nor (side view).] - -The history of the famous stone called the Koh-i-nor, meaning Mound of -Light, is known as far back as the year 1304, when it fell into the -hands of the Mogul emperors, and legend even traces it back some four -thousand years previously. It remained at Delhi until the invasion of -North-West India by Nadir Shah in 1739, when it passed together with an -immense amount of spoil into the hands of the conqueror. At his death -the empire which he had so strenuously founded fell to pieces, and -the great diamond after many vicissitudes came into the possession of -Runjit Singh at Lahore. His successors kept it until upon the fall of -the Sikh power in 1850 it passed to the East India Company, in whose -name it was presented by Lord Dalhousie to Queen Victoria. At this -date the stone still retained its original Indian form, but in 1862 it -was re-cut into the form of a shallow brilliant (Fig. 62), the weight -thereby being reduced from 186-1/16 to 106-1/16 carats. The wisdom -of this course has been severely criticized; the stone has not the -correct shape of a brilliant and is deficient in ‘fire,’ and it has -with the change in shape lost much of its old historical interest. -The Koh-i-nor is the private property of the English Royal Family, the -stone shown in the Tower being a model. It is valued at £100,000. - - - (2) PITT OR REGENT - -[Illustration: FIG. 63.—Pitt or Regent (top view).] - -[Illustration: FIG. 64.—Pitt or Regent (side view).] - -This splendid stone was discovered in 1701 at the famous diamond mines -at Partial, on the Kistna, about 150 miles (240 km.) from Golconda, -and weighed as much as 410 carats in the rough. By devious ways it -came into the hands of Jamchund, a Parsee merchant, from whom it was -purchased by William Pitt, governor of Fort St. George, Madras, for -£20,400. On his return to England Pitt had it cut into a perfect -brilliant (Fig. 63), weighing 163⅞ carats, the operation occupying -the space of two years and costing £5000; more than £7000 is said to -have been realized from the sale of the fragments left over. Pitt -had an uneasy time and lived in constant dread of theft of the stone -until, in 1717, after lengthy negotiations, he parted with it to the -Duc d’Orléans, Regent of France, for the immense sum of three and -three-quarter million francs, about £135,000. With the remainder of the -French regalia it was stolen from the Garde-meuble on August 17, 1792, -in the early days of the French Revolution, but was eventually restored -by the thieves, doubtless because of the impossibility of disposing of -such a stone, at least intact, and it is now exhibited in the Apollo -Gallery of the Louvre at Paris. It measures about 30 millimetres in -length, 25 in width, and 19 in depth, and is valued at £480,000. - - - (3) ORLOFF - -[Illustration: FIG. 65.—Orloff (top view).] - -[Illustration: FIG. 66.—Orloff (side view).] - -One of the finest diamonds existing, this large stone forms the top -of the imperial sceptre of Russia. It is rose-cut (Fig. 65), the base -being a cleavage face, and weighs 194¾ carats. It is said to have -formed at one time one of the eyes of a statue of Brahma which stood -in a temple on the island of Sheringham in the Cavery River, near -Trichinopoli, in Mysore, and to have been stolen by a French soldier -who had somehow persuaded the priests to appoint him guardian of the -temple. He sold it for £2000 to the captain of an English ship, who -disposed of it to a Jewish dealer in London for £12,000. It changed -hands to a Persian merchant, Raphael Khojeh, who eventually sold it to -Prince Orloff for, so it is said, the immense sum of £90,000 and an -annuity of £4000. It was presented by Prince Orloff to Catherine II of -Russia. - - - (4) GREAT MOGUL - -This, the largest Indian diamond known, was found in the Kollur mines, -about the year 1650. Its original weight is said to have been 787½ -carats, but it was so full of flaws that the Venetian, Hortensio -Borgis, then in India, in cutting it to a rose form reduced its weight -to 240 carats. It was seen by Tavernier at the time of his visit -to India, but it has since been quite lost sight of. It has been -identified with both the Koh-i-nor and the Orloff, and it is even -suggested that both these stones were cut from it. - - - (5) SANCY - -The history of this diamond is very involved, and probably two or more -stones have been confused. It may have been the one cut by Berquem for -Charles the Bold, from whose body on the fatal day of Nancy, in 1477, -it was snatched by a marauding soldier. It was acquired by Nicholas -Harlai, Seigneur de Sancy, who sold it to Queen Elizabeth at the close -of the sixteenth century. A hundred years later, in 1695, it was sold -by James II to Louis XIV. The stone in the French regalia, according -to the inventory taken in 1791, weighed 53¾ carats. It was never -recovered after the theft of the regalia in the following year, but -may be identical with the diamond which was in the possession of the -Demidoff family and was sold by Prince Demidoff in 1865 to a London -firm who were said to have been acting for Sir Jamsetjee Jeejeebhoy, a -wealthy Parsee of Bombay. It was shown at the Paris Exhibition of 1867. -It was almond-shaped, and covered all over with tiny facets by Indian -lapidaries. - - - (6) GREAT TABLE - -This mysterious stone was seen by Tavernier at Golconda in 1642, but -has quite disappeared. It weighed 242-3/16 carats. - - - (7) MOON OF THE MOUNTAINS - -This diamond is often confused with the Orloff. It was captured by -Nadir Shah at Delhi, and after his murder was stolen by an Afghan -soldier who disposed of it to an Armenian, by name Shaffrass. It was -finally acquired by the Russian crown for an enormous sum. - - - (8) NIZAM - -A large diamond, weighing 340 carats, belonged to the Nizam of -Hyderabad; it was fractured at the beginning of the Indian Mutiny. -Whether the weight is that previous to fracture or not, there seems to -be no information. - - - (9) DARYA-I-NOR - -This fine diamond, rose-cut and 186 carats in weight, is of the purest -water and merits its title of ‘River of Light.’ It seems to have been -captured by Nadir Shah at Delhi, and is now the largest diamond in the -Persian collection. - - - (10) SHAH - -This fine stone, of the purest water, was presented to the Czar -Nicholas by the Persian prince Chosroes, younger son of Abbas Mirza, in -1843. At that time it still retained three cleavage faces which were -engraved with the names of three Persian sovereigns, and weighed 95 -carats. It was, however, subsequently re-cut with the loss of 9 carats, -and the engraving has disappeared in the process. - - - (11) AKBAR SHAH, OR JEHAN GHIR SHAH - -Once the property of the great Mogul, Akbar, this diamond was engraved -on two faces with Arabic inscriptions by the instructions of his -successor, Jehan. It disappeared, but turned up again in Turkey -under the name of ‘Shepherd’s Stone’; it still retained its original -inscriptions and was thereby recognized. In 1866 it was re-cut, the -weight being reduced from 116 to 71 carats, and the inscriptions -destroyed. The stone was sold to the Gaekwar of Baroda for 3½ lakhs of -rupees (about £23,333). - - - (12) POLAR STAR - -A beautiful, brilliant-cut stone, weighing 40 carats, which is known by -this name, is in the Russian regalia. - - - (13) NASSAK - -The Nassak diamond, which weighed 89¾ carats, formed part of the -Deccan booty, and was put up to auction in London in July 1837. It -was purchased by Emanuel, a London jeweller, who for £7200 shortly -afterwards sold it to the Duke of Westminster, in whose family it still -remains. It was originally pear-shaped, but was re-cut to a triangular -form with a reduction in weight to 78⅝ carats. - - - (14) NAPOLEON - -This diamond was purchased by Napoleon Buonaparte for £8000, and worn -by him at his wedding with Josephine Beauharnais in 1796. - - - (15) CUMBERLAND - -This stone, which weighs 32 carats, was purchased by the city of London -for £10,000 and presented to the Duke of Cumberland after the battle of -Culloden; it is now in the possession of the Duke of Brunswick. - - - (16) PIGOTT - -A fine Indian stone, weighing 47½ carats, this diamond was brought -to England by Lord Pigott in 1775 and sold for £30,000. It came into -the possession of Ali Pacha, Viceroy of Egypt, and was by his orders -destroyed at his death. - - - (17) EUGÉNIE - -This fine stone, weighing 51 carats, was given by the Czarina Catherine -II of Russia to her favourite, Potemkin. It was purchased by Napoleon -III as a bridal gift for his bride, and on his downfall was bought by -the Gaekwar of Baroda. - - - (18) WHITE SAXON - -Square in contour, measuring 1-1/12 in. (28 mm.), and weighing 48¾ -carats, this stone was purchased by Augustus the Strong for a million -thalers (about £150,000). - - - (19) PACHA OF EGYPT - -This 40-carat brilliant was purchased by Ibrahim, Viceroy of Egypt, for -£28,000. - - - (20) STAR OF ESTE - -Though a comparatively small stone, in weight 25½ carats, it is noted -for its perfection of form and quality. It belongs to the Archduke -Franz Ferdinand of Austrian-Este, eldest son of the Archduke Karl -Ludwig. - - - (21) TUSCANY, OR AUSTRIAN YELLOW - -The beauty of this large stone, 133¾ carats in weight, is marred by -the tinge of yellow, which is sufficiently pronounced to impair its -brilliancy; it is a double rose in form. At one time the property of -the Grand Dukes of Tuscany, it is now in the possession of the Emperor -of Austria. King mentions a tale that it was bought at a curiosity -stall in Florence for an insignificant sum, the stone being supposed to -be only yellow quartz. - - - (22) STAR OF THE SOUTH - -This, the largest of the Brazilian diamonds, was discovered at the -mines of Bagagem in July 1853. Perfectly transparent and without tint, -it was dodecahedral in shape and weighed 254½ carats, and was sold in -the rough for £40,000. It was cut as a perfect brilliant, being reduced -in weight to 125½ carats. - - - (23) ENGLISH DRESDEN - -This beautiful stone, which weighed 119½ carats in the rough, was found -at the Bagagem mines, in Brazil, in 1857, and came into the possession -of Mr. E. Dresden. It was cut as a long, egg-shaped brilliant, weighing -76½ carats. - - - (24) STAR OF SOUTH AFRICA - -The first considerable stone to be found in South Africa, it was -discovered at the Vaal River diggings in 1869, and weighed 83½ carats -in the rough. It was cut to a triangular brilliant of 46½ carats. It -was finally purchased by the Countess of Dudley for £25,000. - - - (25) STEWART - -This large diamond, weighing in the rough 288⅜ carats, was found at the -Vaal River diggings in 1872, and was first sold for £6000 and shortly -afterwards for £9000; it was reduced on cutting to 120 carats. Like -many South African stones, it has a faint yellowish tinge. - - - (26) PORTER-RHODES - -This blue-white stone, which weighed 150 carats, was found in a claim -belonging to Mr. Porter-Rhodes in the Kimberley mine in February 1880. - - - (27) IMPERIAL, VICTORIA, OR GREAT WHITE - -This large diamond weighed as much as 457 carats in the rough, and 180 -when cut; it is quite colourless. It was brought to Europe in 1884, and -was eventually sold to the Nizam of Hyderabad for £20,000. - - - (28) DE BEERS - -A pale yellowish stone, weighing 428½ carats, was found in the De Beers -mine in 1888. It was cut to a brilliant weighing 228½ carats, and -was sold to an Indian prince. A still larger stone of similar tinge, -weighing 503¼ carats, was discovered in 1896, and among other large -stones supplied by the same mine may be mentioned one of 302 carats -found in 1884, and another of 409 carats found in early years. - - - (29) EXCELSIOR - -This, which prior to the discovery of the ‘Cullinan,’ was by far the -largest South African stone, was found in the Jagersfontein mine on -June 30, 1893; bluish-white in tint, it weighed 969½ carats. From -it were cut twenty-one brilliants, the larger stones weighing 67⅞, -45-13/16, 45-11/16, 39-3/16, 34, 27⅞, 25⅝, 23-11/16, 16-11/32, 13½ -carats respectively, and the total weight of the cut stones amounting -to 364-3/32 carats. - - - (30) JUBILEE - -Another large stone was discovered in the Jagersfontein mine in -1895. It weighed 634 carats in the rough, and from it was obtained a -splendid, faultless brilliant weighing 239 carats. It was shown at the -Paris Exhibition of 1900. - - - (31) STAR OF AFRICA, OR CULLINAN - -[Illustration: FIG. 67.—Cullinan No. 1.] - -All diamonds pale into insignificance when compared with the colossal -stone that came to light at the Premier mine near Pretoria in the -Transvaal on January 25, 1905. It was first called the ‘Cullinan’ after -Sir T. M. Cullinan, chairman of the Premier Diamond Mine (Transvaal) -Company, but has recently, by desire of King George V, received the -name ‘Star of Africa.’ The rough stone weighed 621·2 grams or 3025¾ -carats (about 1⅓ lb.); it displayed three natural faces (Plate XXV) -and one large cleavage face, and its shape suggested that it was -a portion of an enormous stone more than double its size; it was -transparent, colourless, and had only one small flaw near the surface. -This magnificent diamond was purchased by the Transvaal Government for -£150,000, and presented to King Edward VII on his birthday, November 9, -1907. - -[Illustration: _PLATE XXV_ - -CULLINAN DIAMOND - -(_Natural size_)] - -[Illustration: FIG. 68.—Cullinan No. 2.] - -The Cullinan was entrusted to the famous firm, Messrs. I. J. Asscher -& Co., of Amsterdam, for cutting on January 23, 1908, just three -years after its discovery. On February 10 it was cleaved into two -parts, weighing respectively 1977½ and 1040½ carats, from which the -two largest stones have been cut, one being a pendeloque or drop -brilliant in shape (Fig. 67) and weighing 516½ carats, and the other -a square brilliant (Fig. 68) weighing 309-3/16 carats. The first -has been placed in the sceptre, and the second in the crown of the -regalia. Besides these there are a pendeloque weighing 92 carats, a -square-shaped brilliant 62, a heart-shaped stone 18⅜, two marquises -8-9/16 and 11¼, an oblong stone 6⅝, a pendeloque 4-9/32, and 96 small -brilliants weighing together 7⅜; the total weight of the cut stones -amounts to 1036-5/32 carats. The largest stone has 74 and the second 66 -facets. The work was completed and the stones handed to King Edward in -November 1908. - -Although the Premier mine has yielded no worthy compeer of the -Cullinan, it can, nevertheless, boast of a considerable number of -large stones which but for comparison with that giant would be thought -remarkable for their size, no fewer than seven of them having weights -of over 300 carats, viz. 511, 487¼, 458¾, 391½, 373, 348, and 334 -carats. - - - (32) STAR OF MINAS - -This large diamond, which was found in 1911 at the Bagagem mines, Minas -Geraes, Brazil, had the shape of a dome with a flat base, and weighed -in the rough 35·875 grams (174¾ carats). - - • • • • • - -The large stone called the ‘Braganza,’ in the Portuguese regalia, which -is supposed to be a diamond, is probably a white topaz; it weighs 1680 -carats. The Mattan stone, pear-shaped and weighing 367 carats, which -was found in the Landak mines near the west coast of Borneo in 1787, is -suspected to be quartz. - - - COLOURED DIAMONDS - - - (1) HOPE - -[Illustration: FIG. 69.—Hope.] - -The largest of coloured diamonds, the Hope, weighs 44⅛ carats, and has -a steely- or greenish-blue, and not the royal-blue colour of the glass -models supposed to represent it. It is believed to be a portion of a -drop-form stone (_d’un beau violet_) which was said to have been found -at the Kollur mines, and was secured by Tavernier in India in 1642 and -sold by him to Louis XIV in 1668; it then weighed 67 carats. This stone -was stolen with the remainder of the French regalia in 1792 and never -recovered. In 1830 the present stone (Fig. 69) was offered for sale -by Eliason, a London dealer, and was purchased for £18,000 by Thomas -Philip Hope, a wealthy banker and a keen collector of gems. Probably -the apex of the original stone had been cut off, reducing it to a -nearly square stone. The slight want of symmetry of the present stone -lends confirmation to this view, and two other blue stones are known, -which, together with the Hope, make up the weight of the original -stone. At the sale of the Hope collection at Christie’s in 1867 the -blue diamond went to America. In 1908 the owner disposed of it to Habib -Bey for the enormous sum of £80,000. It was put up to auction in Paris -in 1909, and bought by Rosenau, the Paris diamond merchant, for the -comparatively small sum of 400,000 francs (about £16,000), and was sold -in January 1911 to Mr. Edward M’Lean for £60,000. The stone is supposed -to bring ill-luck in its train, and its history has been liberally -embellished with fable to establish the saying. - - - (2) DRESDEN - -A beautiful apple-green diamond, faultless, and of the purest water, is -contained in the famous Green Vaults of Dresden. It weighs 40 carats, -and was purchased by Augustus the Strong in 1743 for 60,000 thalers -(about £9000). - - - (3) PAUL I - -A fine ruby-red diamond, weighing 10 carats, is included among the -Russian crown jewels. - - - (4) TIFFANY - -The lovely orange brilliant, weighing 125⅜ carats, which is in the -possession of Messrs. Tiffany & Co., the well-known jewellers of New -York, was discovered in the Kimberley mine in 1878. - - - - - CHAPTER XIX - - CORUNDUM - - (_Sapphire_, _Ruby_) - - -Ranking in hardness second to diamond alone, the species known to -science as corundum and widely familiar by the names of its varieties, -sapphire and ruby, holds a pre-eminent position among coloured -gem-stones. The barbaric splendour of ruby (Plate I, Fig. 13) and the -glorious hue of sapphire (Plate I, Fig. 11) are unsurpassed, and it -is remarkable that the same species should boast such different, but -equally magnificent, tints. They, however, by no means exhaust the -resources of this variegated species. Fine yellow stones (Plate I, -Fig. 12), which compare with topaz in colour and are its superior in -hardness, and brilliant colourless stones, which are unfortunately -deficient in ‘fire’ and cannot therefore approach diamond, are to -be met with, besides others of less attractive hues, purple, and -yellowish, bluish, and other shades of green. Want of homogeneity in -the coloration of corundum is a frequent phenomenon; thus, the purple -stones on close examination are found to be composed of alternate blue -and red layers, and stones showing patches of yellow and blue colour -are common. Owing to the peculiarity of their interior arrangement -certain stones display when cut _en cabochon_ a vivid six-rayed star of -light (Plate I, Fig. 15). Sapphire and ruby share with diamond, pearl, -and emerald the first rank in jewellery. They are popular stones, -especially in rings; their comparative rarity in large sizes, apart -from the question of expense, prevents their use in the bigger articles -of jewellery. The front of the stones is usually brilliant-cut and the -back step-cut, but Indian lapidaries often prefer to cover the stone -with a large number of triangular facets, especially if the stone be -flawed; star-stones are cut more or less steeply _en cabochon_. - -In composition corundum is alumina, oxide of aluminium, corresponding -to the formula Al_{2}O_{3}, but it usually contains in addition small -quantities, rarely more than 1 per cent., of ferric oxide, chromic -oxide, and perhaps other metallic oxides. When pure, it is colourless; -the splendid tints which are its glory have their origin in the minute -traces of the other oxides present. No doubt chromic oxide is the -cause of the ruddy hue of ruby, since it is possible, as explained -above (p. 117), closely to imitate the ruby tint by this means, but -nothing approaching so large a percentage as 2½ has been detected in -a natural stone. The blue colour of sapphire may be due to titanic -oxide, and ferric oxide may be responsible for the yellow hue of -the ‘oriental topaz,’ as the yellow corundum is termed. Sapphires, -when of considerable size, are rarely uniform in tint throughout the -stone. Alternations of blue and red zones, giving rise to an apparent -purple or violet tint, and the conjunction of patches of blue and -yellow are common. Perfectly colourless stones are less common, a -slight bluish tinge being usually noticeable, but they are not in much -demand because, on account of their lack of ‘fire,’ they are of little -interest when cut. The tint of the red stones varies considerably in -depth; jewellers term them, when pale, pink sapphires, but, of course, -no sharp distinction can be drawn between them and rubies. The most -highly prized tint is the so-called pigeon’s blood, a shade of red -slightly inclined to purple. The prices for ruby of good colour run -from about 25s. a carat for small stones to between £60 and £80 a -carat for large stones, and still higher for exceptional rubies. The -taste in sapphires has changed of recent times. Formerly the deep blue -was most in demand, but now the lighter shade, that resembling the -colour of corn-flower, is preferred, because it retains a good colour -in artificial light. Large sapphires are more plentiful than large -rubies, and prices run lower; even for large perfect stones the rate -does not exceed £30 a carat. Large and uniform ‘oriental topazes’ are -comparatively common, and realize moderate prices, about 2s. to 30s. -a carat according to quality and size. Green sapphires are abundant -from Australia, but their tint, a kind of deep sage-green, is not very -pleasing. Brown stones with a silkiness of structure are also known. - -The name of the species comes through the French _corindon_ from an -old Hindu word, _korund_, of unknown significance, and arose from the -circumstance that the stones which first found their way to Europe came -from India. At the present day the word corundum is applied in commerce -to the opaque stones used for abrasive purposes, to distinguish the -purer material from emery, which is corundum mixed with magnetite and -other heavy stones of lower hardness. The origin of the word sapphire, -which means blue, has been discussed in an earlier chapter (p. 110). -Jewellers use it in a general sense for all corundum except ruby. Ruby -comes from the Latin _ruber_, red. The prefix ‘oriental’ (p. 111) -is often used to distinguish varieties of corundum, since it is the -hardest of ordinary coloured stones and the finest gem-stones in early -days reached Europe by way of the East. - -Corundum crystallizes either in six-sided prisms terminated by flat -faces (Plate I, Fig. 10), which are often triangularly marked, or with -twelve inclined faces, six above and six below, meeting in a girdle -(Plate I, Fig. 14). Ruby favours the former and the other varieties -the latter type. A fine crystal of ruby—the ‘Edwardes,’ so named by -the donor, John Ruskin, after Sir Herbert Edwardes—which weighs 33·5 -grams (163 carats), is exhibited in the Mineral Gallery of the British -Museum (Natural History), and is tilted in such a way that the light -from a neighbouring window falls on the large basal face, and reveals -the interesting markings that nature has engraved on it. From its type -of symmetry corundum is doubly refractive with a direction of single -refraction running parallel to the edge of the prism. Owing to the -relative purity of the chemical composition the refractive indices -are very constant; the ordinary index ranges from 1·766 to 1·774 and -the extraordinary index from 1·757 to 1·765, the double refraction -remaining always the same, 0·009. The amount of colour-dispersion is -small, and therefore colourless corundum displays very little ‘fire.’ -The difference between the indices for red and blue light is, however, -sufficiently great that the base of a ruby may be left relatively -thicker than that of a sapphire to secure an equally satisfactory -effect (cf. p. 98)—a point of some importance to the lapidary, since -stones are sold by weight and it is his object to keep the weight as -great as possible. When a corundum is tested on the refractometer in -white light a wide spectrum deliminates the two portions of the field -because of the smallness of the colour-dispersion (cf p. 25). The -dichroism of both ruby and sapphire is marked, the twin colours given -by the former being red and purplish-red, and by the latter blue and -yellowish-blue, the second colour in each instance corresponding to -the extraordinary ray. Tests with the dichroscope easily separate ruby -and sapphire from any other red or blue stone. This character has an -important bearing on the proper mode of cutting the stones. The ugly -yellowish tint given by the extraordinary ray of sapphire should be -avoided by cutting the stone with its table-facet at right angles to -the prism edge, which is the direction of single refraction. Whether -a ruby should be treated in the same way is a moot point. No doubt -if the colour is deep, it is the best plan, because the amount of -absorption of light is thereby sensibly reduced, but otherwise the -delightful nuances distinguishing ruby are best secured by cutting -the table-facet parallel to the direction of single refraction. -Yellow corundum also shows distinct dichroism, but by a variation -more of the depth than of the tint of the colour; the phenomenon is -faint compared with the dichroic effect of a yellow chrysoberyl. The -specific gravity also is very constant, varying only from 3·95 to 4·10; -sapphire is on the whole lighter than ruby. Corundum has the symbol 9 -on Mohs’s scale, but though coming next to diamond it is a very poor -second (cf. p. 79). As is usually the case, the application of heat -tends to lighten the colour of the stones: those of a pale violet or -a yellow colour lose the tint entirely, and the deep violet stones -turn a lovely rose colour. On the other hand the action of radium has, -as was shown by Bordas, an intensifying action on the colour, and -even develops it in a colourless stone. From the latter reaction it -may be inferred that often in an apparently colourless stone two or -more selective influences are at work which ordinarily neutralize one -another, but, being unequally stimulated by the action of radium, they -thereupon give rise to colour. The stellate appearance of asterias -or star-stones—star-ruby and star-sapphire—results from the regular -arrangement either of numerous small channels or of twin-lamellæ in the -stone parallel to the six sides of the prisms; light is reflected from -the interior in the form of a six-rayed star (p. 38). Some stones from -Siam possess a markedly fibrous or silky structure. - -The synthetical manufacture of ruby, sapphire, and other varieties of -corundum has already been described (p. 116). - -Besides its use in jewellery corundum is on account of its hardness of -great service for many other purposes. Small fragments are extensively -employed for the bearing parts of the movements of watches, and both -the opaque corundum and the impure kind known as emery are in general -use for grinding and polishing softer stones, and steel and other -metal-work. - -The world’s supply of fine rubies is drawn almost entirely from the -famous ruby mines near Mogok, situated about 90 miles (145 km.) in -a north-easterly direction from Mandalay in Upper Burma and at an -elevation of about 4000 ft. (1200 m.) above sea-level. It is from this -district that the stones of the coveted carmine-red, the so-called -‘pigeon’s blood,’ colour are obtained. The ruby occurs in a granular -limestone or calcite in association with the spinel of nearly the -same appearance—the ‘balas-ruby,’ oriental topaz (yellow corundum), -tourmaline, and occasionally sapphire. Some stones are found in -the limestone on the sides of the hills, but by far the largest -quantity occur in the alluvial deposits, both gravel and clay, in the -river-beds; the ruby ground is locally known as ‘_byon_.’ The stones -are as a rule quite small, averaging only about four to the carat. -Before the British annexation of the country in 1885 the mines were a -monopoly of the Burmese sovereigns and were worked solely under royal -licence. They are known to be of great antiquity, but otherwise their -early history is a mystery. It is said that an astute king secured the -priceless territory in 1597 from the neighbouring Chinese Shans in -exchange for a small and unimportant town on the Irrawaddy; if that -be so, he struck an excellent bargain. The mines were allotted to -licensed miners, _twin-tsas_ (eaters of the mine) as they were called -in the language of the country, who not only paid for the privilege, -but were compelled to hand over to the king all stones above a certain -weight. As might be anticipated this injunction caused considerable -trouble, and the royal monopolists constantly suspected the miners of -evading the regulation by breaking up stones of exceptional size; from -subsequent experience, it is probable that large stones were in reality -seldom found. Since 1887 the mines have been worked by arrangement with -the Government of India by the Ruby Mines, Ltd., an English company. -Its career has been far from prosperous, but during recent years, in -consequence of the improved methods of working the mines and of the -more generous terms afterwards accorded by the Government, greater -success has been experienced; the future is, however, to some extent -clouded by the advent of the synthetical stone, which has even made its -way out to the East. - -Large rubies are far from common, and such as were discovered in the -old days were jealously hoarded by the Burmese sovereigns. According to -Streeter the finest that ever came to Europe were a pair brought over -in 1875, at a time when the Burmese king was pressed for money. One, -rich in colour, was originally cushion-shaped and weighed 37 carats; -the other was a blunt drop in form and weighed 47 carats. Both were cut -in London, the former being reduced to 32-5/16 carats and the latter -to 38-9/16 carats, and were sold for £10,000 and £20,000 respectively. -A colossal stone, weighing 400 carats, is reported to have been found -in Burma; it was broken into three pieces, of which two were cut and -resulted in stones weighing 70 and 45 carats respectively, and the -third was sold uncut in Calcutta for 7 lakhs of rupees (£46,667). The -finder of another large stone broke it into two parts, which after -cutting weighed 98 and 74 carats respectively; he attempted in vain to -evade the royal acquisitiveness, by giving up the larger stone to the -king and concealing the other. A fine stone, known by the formidable -appellation of ‘Gnaga Boh’ (Dragon Lord), weighed 44 carats in the -rough and 20 carats after cutting. Since the mines were taken over -by the Ruby Mines, Ltd., a few large stones have been discovered. A -beautiful ruby was found in the Tagoungnandaing Valley, and weighed -18½ carats in the rough and 11 carats after cutting; perfectly clear -and of splendid colour, it was sold for £7000, but is now valued at -£10,000. Another, weighing 77 carats in the rough, was found in 1899, -and was sold in India in 1904 for 4 lakhs of rupees (£26,667). A stone, -weighing 49 carats, was discovered in 1887, and an enormous one, -weighing as much as 304 carats, in 1890. - -The ruby, as large as a pigeon’s egg, which is amongst the Russian -regalia was presented in 1777 to the Czarina Catherine by Gustav III of -Sweden when on a visit to St. Petersburg. The large red stone in the -English regalia which was supposed to be a ruby is a spinel (cf p. 206). - -Comparatively uncommon as sapphires are in the Burma mines a faultless -stone, weighing as much as 79½ carats, has been discovered there. - -Good rubies, mostly darker in colour than the Burmese stones, are -found in considerable quantity near Bangkok in Siam, Chantabun -being the centre of the trade, where, just as in Burma, they are -intimately associated with the red spinel. Because of the difference -in tint and the consequent difference in price, jewellers draw a -distinction between Burma and Siam rubies; but that, of course, does -not signify any specific difference between them. Siam is, however, -most distinguished as the original home of splendid sapphires. The -district of Bo Pie Rin in Battambang produces, indeed, more than -half the world’s supply of sapphires. In the Hills of Precious -Stones, such being the meaning of the native name for the locality, a -number of green corundums are found. Siam also produces brown stones -characterized by a peculiar silkiness of structure. Rubies are found in -Afghanistan at the Amir’s mines near Kabul and also to the north of the -lapis lazuli mines in Badakshan. - -The conditions in Ceylon are precisely the converse of those obtaining -in Burma; sapphire is plentiful and ruby rare in the island. They are -found in different rocks, sapphire occurring with garnet in gneiss, and -ruby accompanying spinel in limestone, but they come together in the -resulting gravels, the principal locality being the gem-district near -Ratnapura in the south of the island. The largest uncut ruby discovered -in Ceylon weighed 42½ carats; it had, however, a decided tinge of -blue in it. Ceylon is also noted for the magnificent yellow corundum, -‘oriental topaz,’ or, as it is locally called, ‘king topaz,’ which it -produces. - -Beautiful sapphires occur in various parts of India, but particularly -in the Zanskar range of the north-western Himalayas in the state of -Kashmir, where they are associated with brown tourmaline. Probably most -of the large sapphires known have emanated from India. By far the most -gigantic ever reported is one, weighing 951 carats, said to have been -seen in 1827 in the treasury of the King of Ava. The collection at the -Jardin des Plantes contains two splendid rough specimens; one, known as -the ‘Rospoli,’ is quite flawless and weighs 132-1/16 carats, and the -other is 2 inches in length and 1½ inches in thickness. The Duke of -Devonshire possesses a fine cut stone, weighing 100 carats, which is -brilliant-cut above and step-cut below the girdle. An image of Buddha, -which is cut out of a single sapphire, is exhibited, mounted on a gold -pin, in the Mineral Gallery of the British Museum (Natural History). - -For some years past a large quantity of sapphires have come into the -market from Montana, U.S.A., especially from the gem-district about -twelve miles west of Helena. The commonest colour is a bluish green, -generally pale, but blue, green, yellow and occasionally red stones are -also found; they are characterized by their almost metallic lustre. -With them are associated gold, colourless topaz, kyanite, and a -beautiful red garnet which is found in grains and usually mistaken for -ruby. Rubies are also found in limestone at Cowee Creek, North Carolina. - -Blue and red corundum, of rather poor quality, has come from the -Sanarka River, near Troitsk, and from Miask, in the Government of -Orenburg, Russia, and similar stones have been known at Campolongo, St. -Gothard, Switzerland. - -The prolific gem-district near Anakie, Queensland, supplies examples of -every known variety of corundum except ruby; blue, green, yellow, and -parti-coloured stones, and also star-stones, are plentiful. Leaf-green -corundum is known father south, in Victoria. The Australian sapphire is -too dark to be of much value. - -Small rubies and sapphires are found in the gem-gravels near the -Somabula Forest, Rhodesia. - - - - - CHAPTER XX - - BERYL - - (_Emerald_, _Aquamarine_, _Morganite_) - - -The species to be considered in this chapter includes the varieties -emerald and aquamarine, as well as what jewellers understand by beryl. -It has many incontestable claims on the attention of all lovers of the -beautiful in precious stones. The peerless emerald (Plate I, Fig. 5), -which in its verdant beauty recalls the exquisite lawns that grace the -courts and quadrangles of our older seats of learning, ranks to-day -as the most costly of jewels. Its sister stone, the lovely aquamarine -(Plate I, Fig. 4), which seems to have come direct from some mermaid’s -treasure-house in the depths of a summer sea, has charms not to be -denied. Pliny, speaking of this species, truly says, “There is not a -colour more pleasing to the eye”; yet he knew only the comparatively -inferior stones from Egypt, and possibly from the Ural Mountains. -Emeralds are favourite ring-stones, and would, no doubt, be equally -coveted for larger articles of jewellery did not the excessive cost -forbid, and nothing could be more attractive for a central stone than -a choice aquamarine of deep blue-green hue. Emeralds are usually -step-cut, though Indian lapidaries often favour the _en cabochon_ -form; aquamarines, on the other hand, are brilliant-cut in front and -step-cut at the back. - -Beryl, to use the name by which the species is known to science, is -essentially a silicate of aluminium and beryllium corresponding to the -formula, Be_{3}Al_{2}(SiO_{3})_{6}. The beryllia is often partially -replaced by small amounts of the alkaline earths, caesia, potash, soda, -and lithia, varying from about 1½ per cent. in beryl from Mesa Grande -to nearly 5 in that from Pala and Madagascar, and over 6, of which 3·6 -is caesia, in beryl from Hebron, Maine; also, as usual, chromic and -ferric oxides take the place of a little alumina; from 1 to 2 per cent. -of water has been found in emerald. The element beryllium was, as its -name suggests, first discovered in a specimen of this species, the -discovery being made in 1798 by the chemist Vauquelin; it is also known -as glucinum in allusion to the sweet taste of its salts. - -When pure, beryl is colourless, but it is rarely, if ever, free -from a tinge of blue or green. The colour is usually some shade of -green—grass-green, of that characteristic tint which is in consequence -known as emerald-green, or blue-green, yellowish green (Plate I, Fig. -6), and sometimes yellow, pink, and rose-red. The peculiar colour of -emerald is supposed to be caused by chromic oxide, small quantities -of which have been detected in it by chemical analysis; moreover, -experiment shows that glass containing the same percentage amount of -chromic oxide assumes the same splendid hue. Emerald, on being heated, -loses water, but retains its colour unimpaired, which cannot therefore -be due, as has been suggested, to organic matter. The term aquamarine -is applied to the deep sea-green and blue-green stones, and jewellers -restrict the term beryl to paler shades and generally other colours, -such as yellow, golden, and pink, but Kunz has recently proposed the -name morganite to distinguish the beautiful rose beryl such as is -found in Madagascar. The varying shades of aquamarine are due to the -influence of the alkaline earths modified by the presence of ferric -oxide or chromic oxide; the beautiful blushing hue of morganite is no -doubt caused by lithia. - -[Illustration: FIG. 70.—Emerald Crystal.] - -The name of the species is derived from the Greek βήρυλλος, an ancient -word, the meaning of which has been lost in the mists of time. The -Greek word denoted the same species in part as that now understood -by the name. Emerald is derived from a Persian word which appeared -in Greek as σμάραγδος, and in Latin as _smaragdus_; it originally -denoted chrysocolla, or similar green stone, but was transferred upon -the introduction of the deep-green beryl from Upper Egypt. The name -aquamarine was suggested by Pliny’s exceedingly happy description of -the stones “which imitate the greenness of the clear sea,” although it -was not actually used by him. That emerald and beryl were one species -was suspected by Pliny, but the identity was not definitely established -till about a century ago. Morganite is named after John Pierpont Morgan. - -The natural crystals have the form of a six-sided prism, and in the -case of emerald (Fig. 70, and Plate I, Fig. 8) invariably, if whole, -end in a single face at right angles to the length of the prism; -aquamarines have in addition a number of small inclined faces, and -stones from both Russia and Brazil often taper owing to the effects -of corrosion. The sixfold character of the crystalline symmetry -necessarily entails that the double refraction, which is small in -amount, 0·006, is uniaxial in character, and, since the ordinary -is greater than the extraordinary refractive index, it is negative -in sign. The values of the indices range between 1·567 and 1·590, -and 1·572 and 1·598 respectively, in the two cases, the pink beryl -possessing the highest values. The dichroism is distinct in the South -American emerald, the twin colours being yellowish and bluish green, -but otherwise is rather faint. The specific gravity varies between -2·69 and 2·79, and is therefore a little higher than that of quartz. -If, therefore, a beryl and a quartz be floating together in a tube -containing a suitable heavy liquid, the former will always be at a -sensibly lower level (cf. Fig. 32). The hardness varies from 7½ to -8, emerald being a little softer than the other varieties. There is -no cleavage, but like most gem-stones beryl is very brittle, and can -easily be fractured. Stones rendered cloudy by fissures are termed -‘mossy.’ When heated before the blowpipe beryl is fusible with -difficulty; it resists the attack of hydrofluoric acid as well as of -ordinary acids. - -In all probability the whole of the emeralds known in ancient times -came from the so-called Cleopatra emerald mines in Upper Egypt. For -some reason they were abandoned, and their position was so completely -lost that in the Middle Ages it was maintained that emeralds had never -been found in Egypt at all, but had come from America by way of the -East. All doubts were set at rest by the re-discovery of the mines -early last century by Cailliaud, who had been sent by the Viceroy of -Egypt to search for them. They were, however, not much worked, and -after a few years were closed again, and were re-opened only about ten -years ago. The principal mines are at Jebel Zabara and at Jebel Sikait -in northern Etbai, about 10 miles (16 km.) apart and distant about 15 -miles (24 km.) from the Red Sea, lying in the range of mountains that -run for a long distance parallel to the west coast of the Red Sea and -rise to over 1800 feet (550 m.) above sea-level. There are numerous -signs of considerable, but primitive, workings at distinct periods. -Both emeralds and beryls are found in micaceous and talcose schists. -The emeralds are not of very good quality, being cloudy and rather -light in colour. Finer emeralds have been found in a dark mica-schist, -together with other beryllium minerals, chrysoberyl and phenakite, and -also topaz and tourmaline on the Asiatic side of the Ural Mountains, -near the Takowaja River, which flows into the Bolshoi Reft River, one -of the larger tributaries of the Pyschma River, about fifty miles (80 -km.) east of Ekaterinburg, a town which is chiefly concerned with the -mining and cutting of gem-stones. The mine was accidentally discovered -by a peasant, who noticed a few green stones at the foot of an uprooted -tree in 1830. Two years later the mine was regularly worked, and -remained open for twenty years, when it was closed. It has recently -been re-opened owing to the high rates obtaining for emeralds. Very -large crystals have been produced here, but in colour they are much -inferior to the South American stones; small Siberian emeralds, on the -other hand, are of better colour than small South American emeralds, -the latter being not so deep in tint. Emeralds have been found in a -similar kind of schist at Habachtal, in the Salzburg Alps. About thirty -years ago well-formed green stones were discovered with hiddenite at -Stony Point, Alexander County, in North Carolina, but not much gem -material has come to light. - -The products of none of the mines that have just been mentioned can -on the whole compare with the beautiful stones which have come from -South America. At the time when the Spaniards grimly conquered Peru -and ruthlessly despoiled the country of the treasures which could be -carried away, immense numbers of emeralds—some of almost incredible -size—were literally poured into Spain, and eventually found their -way to other parts of Europe. These stones were known as Spanish or -Peruvian emeralds, but in all probability none of them were actually -mined in Peru. Perhaps the most extraordinary were the five choice -stones which Cortez presented to his bride, the niece of the Duke de -Bejar, thereby mortally offending the Queen, who had desired them for -herself, and which were lost in 1529 when Cortez was shipwrecked on -his disastrous voyage to assist Charles V at the siege of Algiers. -All five stones had been worked to divers fantastic shapes. One was -cut like a bell with a fine pearl for a tongue, and bore on the rim, -in Spanish, “Blessed is he who created thee.” A second was shaped -like a rose, and a third like a horn. A fourth was fashioned like a -fish, with eyes of gold. The fifth, which was the most valuable and -the most remarkable of all, was hollowed out into the form of a -cup, and had a foot of gold; its rim, which was formed of the same -precious metal, was engraved with the words, “Inter natos mulierum non -surrexit major.” As soon as the Spaniards had seized nearly all the -emeralds that the natives had amassed in their temples or for personal -adornment, they devoted their attention to searching for the source -of these marvels of nature, and eventually in 1558 they lighted by -accident upon the mines in what is now the United States of Colombia, -which have been worked almost continuously since that time. Since the -natives, who naturally resented the gross injustice with which they -had been treated, and penetrated the greed that prompted the actions -of the Spaniards, hid all traces of the mines, and refused to give any -information as to their position, it is possible that other emerald -mines may yet be found. The present mines are situated near the -village of Muzo, about 75 miles (120 km.) north-north-west of Bogota, -the capital of Colombia. The emeralds occur in calcite veins in a -bituminous limestone of Cretaceous age. The Spaniards formerly worked -the mines by driving adits through the barren rock on the hillsides to -the gem-bearing veins, but at the present day the open cut method of -working is employed. A plentiful supply of water is available, which -is accumulated in reservoirs and allowed at the proper time to sweep -the debris of barren rock away into the Rio Minero, leaving the rock -containing the emeralds exposed. Stones, of good quality, which are -suited for cutting, are locally known as _canutillos_, inferior stones, -coarse or ill-shaped, being called _morallons_. - -Emerald, unlike some green stones, retains its purity of colour in -artificial light; in fact, to quote the words of Pliny, “For neither -sun nor shade, nor yet the light of candle, causeth to change and lose -their lustre.” Many are the superstitions that have been attached to -it. Thus it was supposed to be good for the eyes, and as Pliny says, -“Besides, there is not a gem or precious stone that so fully possesseth -the eye, and yet never contenteth it with satiety. Nay, if the sight -hath been wearied and dimmed by intentive poring upon anything else, -the beholding of this stone doth refresh and restore it again.” The -idea that it was fatal to the eyesight of serpents appears in Moore’s -lines— - - “Blinded like serpents when they gaze - Upon the emerald’s virgin blaze.” - -The crystals occur attached to the limestone, and are therefore never -found doubly terminated. The crystal form is very simple, merely a -hexagonal prism with a flat face at the one end at right angles to it. -They are invariably flawed, so much so that a flawless emerald has -passed into proverb as unattainable perfection. The largest single -crystal which is known to exist at the present day is in the possession -of the Duke of Devonshire (Fig. 71). In section it is nearly a regular -hexagon, about 2 inches (51 mm.) in diameter from side to side, and -the length is about the same; its weight is 276·79 grams (9¾ oz. Av., -or 1347 carats). It is of good colour, but badly flawed. It was given -to the Duke of Devonshire by Dom Pedro of Brazil, and was exhibited -at the Great Exhibition of 1851. A fine, though much smaller crystal, -but of even better colour, which weighs 32·2 grams (156½ carats), and -measures 1⅛ inch (28 mm.) in its widest cross-diameter, and about the -same in length, was acquired with the Allan-Greg collection by the -British Museum, and is exhibited in the Mineral Gallery of the British -Museum (Natural History). The finest cut emerald is said to be one -weighing 30 carats, which belongs to the Czar of Russia. A small, but -perfect and flawless, faceted emerald, which is set in a gold hoop, is -also in the British Museum (Natural History). It is shown, without the -setting, about actual size, on Plate I, Fig. 5. - -[Illustration: FIG. 71.—Duke of Devonshire’s Emerald. - -(Natural size.)] - -The ever great demand and the essentially restricted supply have forced -the cost of emeralds of good quality to a height that puts large stones -beyond the reach of all but a privileged few who have purses deep -enough. The rate per carat may be anything from £15 upwards, depending -upon the purity of the colour and the freedom from flaws, but it -increases very rapidly with the size, since flawless stones of more -than 4 carats or so in weight are among the rarest of jewels; a perfect -emerald of 4 carats may easily fetch £1600 to £2000. It seems anomalous -to say that it has never been easier to procure fine stones than during -recent years, but the reason is that the high prices prevailing have -tempted owners of old jewellery to realize their emeralds. On the other -hand, pale emeralds are worth only a nominal sum. - -The other varieties of beryl are much less rare, and, since they -usually attain to more considerable, and sometimes even colossal, size, -far larger stones are obtainable. An aquamarine, particularly of good -deep blue-green colour, is a stone of great beauty, and it possesses -the merit of preserving its purity of tint in artificial light. It is a -favourite stone for pendants, brooches, and bracelets, and all purposes -for which a large blue or green stone is desired. The varying tints are -said to be due to the presence of iron in different percentages, and -possibly in different states of oxidation. Unlike emerald, the other -varieties are by no means so easily recognized by their colour. Blue -aquamarines may easily be mistaken for topaz, or vice versa, and the -yellow beryl closely resembles other yellow stones, such as quartz, -topaz, or tourmaline. Stones which are colourless or only slightly -tinted command little more than the price of cutting, but the price of -blue-green stones rapidly advances with increasing depth of tint up -to £2 a carat. The enormous cut aquamarine which is exhibited in the -Mineral Gallery of the British Museum (Natural History), affords some -idea of the great size such stones reach; a beautiful sea-green in -colour, it weighs 179·5 grams (875 carats), and is table-cut with an -oval contour. - -The splendid six-sided columns which have been discovered in various -parts of Siberia are among the most striking specimens in any large -mineral collection. The neighbourhood of Ekaterinburg in the Urals -is prolific in varieties of aquamarine; especially at Mursinka have -fine stones been found, in association with topaz, amethyst, and -schorl, the black tourmaline. Good stones also occur in conjunction -with topaz at Miask in the Government of Orenburg. It is found in the -gold-washings of the Sanarka River, in the Southern Urals, but the -stones are not fitted for service as gems. Magnificent blue-green and -yellow aquamarines are associated with topaz and smoky quartz in the -granite of the Adun-Tschilon Mountains, near Nertschinsk, Transbaikal. -Stones have also been found at the Urulga River in Siberia. Most -of the bluish-green aquamarines which come into the market at the -present time have originated in Brazil, particularly in Minas Novas, -Minas Geraes, where clear, transparent stones, of pleasing colour, in -various shades, are found in the utmost profusion; beautiful yellow -stones also occur at the Bahia mines. Aquamarine was obtained in very -early times in Coimbatore District, Madras, India, and yellow beryl -comes from Ceylon. Fine blue crystals occur in the granite of the -Mourne Mountains, Ireland, but they are not clear enough for cutting -purposes; similar stones are found also at Limoges, Haute Vienne, -France. Aquamarines of various hues abound in several places in the -United States, among the principal localities being Stoneham in Maine, -Haddam in Connecticut, and Pala and Mesa Grande in San Diego County, -California. The last-named state is remarkable for the numerous stones -of varying depth of salmon-pink that have been found there. It is, -however, surpassed by Madagascar, which has recently produced splendid -stones of perfect rose-red tint and of the finest gem quality, some of -them being nearly 100 carats in weight. These stones, which have been -assigned a special name, morganite (cf. _supra_), are associated with -tourmaline and kunzite. Pink and yellow beryls and deep blue-green -aquamarines occur in the island in quantity. The pink beryls from -California are generally pale or have a pronounced salmon tint, and -seldom approach the real rose-red colour of morganite; one magnificent -rose-red crystal, weighing nearly 9 lb. (4·05 kg.), has, however, been -recently discovered in San Diego County, California, and is now in the -British Museum (Natural History). Blue-green beryl, varying in tint -from almost colourless to an emerald-green, occurs with tin-stone and -topaz about 9 miles (14½ km.) north-east of Emmaville in New South -Wales, Australia. - -Probably the largest and finest aquamarine crystal ever seen was one -found by a miner on March 28, 1910, at a depth of 15 ft. (5 m.) in a -pegmatite vein at Marambaya, near Arassuahy, on the Jequitinhonha -River, Minas Geraes, Brazil. It was greenish blue in colour, and a -slightly irregular hexagonal prism, with a flat face at each end, in -form; it measured 19 in. (48·5 cm.) in length and 16 in. (41 cm.) in -diameter, and weighed 243 lb. (110·5 kg.); and its transparency was so -perfect that it could be seen through from end to end (Plate XXVI). The -crystal was transported to Bahia, and sold for $25,000 (£5133). - -[Illustration: _PLATE XXVI_ - -LARGE AQUAMARINE CRYSTAL (_one-sixth natural size_), FOUND AT -MARAMBAYA, MINAS GERAES, BRAZIL] - - - - - PART II—SECTION B - - SEMI-PRECIOUS STONES - - - - - CHAPTER XXI - - TOPAZ - - -Topaz is the most popular yellow stone in jewellery, and often -forms the principal stone in brooches or pendants, especially in -old-fashioned articles. It is a general idea that all yellow stones -are topazes, and all topazes are yellow; but neither statement is -correct. A very large number of yellow stones that masquerade as -topaz are really the yellow quartz known as citrine. The latter is, -indeed, almost universally called by jewellers topaz, the qualification -‘Brazilian’ being used by them to distinguish the true topaz. Many -species besides those mentioned yield yellow stones. Thus corundum -includes the beautiful ‘oriental topaz’ or yellow sapphire, and yellow -tourmalines are occasionally met with; the yellow chrysoberyl always -has a greenish tinge. Topaz is generally brilliant-cut in front and -step-cut at the back, and the table facet is sometimes rounded, but -the colourless stones are often cut as small brilliants; it takes an -excellent and dazzling polish. - -Topaz is a silicate of aluminium corresponding to the formula -[Al(F,OH)]_{2}SiO_{4}, which was established in 1894 by Penfield and -Minor as the result of careful research. Contrary to the general -idea, topaz is usually colourless or very pale in tint. Yellow hues -of different degrees, from pale to a rich sherry tint (Plate I, Fig. -9), are common, and pure pale blue (Plate I, Fig. 7) and pale green -stones, which often pass as aquamarine, are far from rare. Natural, -red and pink, stones are very seldom to be met with. It is, however, a -peculiarity of the brownish-yellow stones from Brazil that the colour -is altered by heating to a lovely rose-pink. Curiously, the tint is not -apparent when the stone is hot, but develops as it cools to a normal -temperature; the colour seems to be permanent. Such stones are common -in modern jewellery. Although the change in colour is accompanied by -some slight rearrangement of the constituent molecules, since such -stones are invariably characterized by high refraction and pronounced -dichroism, the crystalline symmetry, however, remaining unaltered, -the cause must be attributed to some change in the tinctorial agent, -probably oxidation. The yellow stones from Ceylon, if treated in a -similar manner, lose their colour entirely. The pale yellow-brown -stones from Russia fade on prolonged exposure to strong sunlight, for -which reason the superb suite of crystals from the Urulga River, which -came with the Koksharov collection to the British Museum, are kept -under cover. - -The name of the species is derived from _topazion_ (τοπάζειν, to -seek), the name given to an island in the Red Sea, which in olden -times was with difficulty located, but it was applied by Pliny and -his contemporaries to the yellowish peridot found there. The term -was applied in the Middle Ages loosely to any yellow stone, and was -gradually applied more particularly to the stone that was then more -prevalent, the topaz of modern science. As has already been pointed out -(p. 111), the term is still employed in jewellery to signify any yellow -stone. The true topaz was probably included by Pliny under the name -_chrysolithus_. - -[Illustration: FIG. 72.—Topaz Crystal.] - -The symmetry is orthorhombic, and the crystals are prismatic in shape -and terminated by numerous inclined faces, and usually by a large -face perpendicular to the prism edge (Fig. 72). Topaz cleaves with -great readiness at right angles to the prism edge; owing to its facile -cleavage, flaws are easily started, and caution must be exercised -not to damage a stone by knocking it against hard and unyielding -substances. The dichroism of a yellow topaz is always perceptible, -one of the twin colours being distinctly more reddish than the other, -and the phenomenon is very marked in the case of stones the colour of -which has been artificially altered to pink. The values of the least -and the greatest of the principal indices of refraction vary from 1·615 -to 1·629, and from 1·625 to 1·637, respectively, the double refraction -being about 0·010 in amount, and positive in sign. The high values -correspond to the altered stones. The specific gravity, the mean value -of which is 3·55 with a variation of 0·05 on either side, is higher -than would be expected from the refractivity. A cleavage flake exhibits -in convergent polarized light a wide-angled biaxial picture, the ‘eyes’ -lying outside the field of view. The relation of the principal optical -directions and the directions of single refraction to the crystal -are shown in Fig. 27. The hardness is 8 on Mohs’s scale, and in this -character it is surpassed only by chrysoberyl, corundum, and diamond. -Topaz is pyro-electric, in which respect tourmaline alone exceeds it, -and it may be strongly electrified by friction. - -Although the range of refraction overlaps that of tourmaline, there -is no risk of confusion, because the latter has nearly thrice the -amount of double refraction (cf. p. 29). Apart from the difference in -refraction, a yellow topaz ought never to be confused with a yellow -quartz, because the former sinks, and the latter floats in methylene -iodide. The same test distinguishes topaz from beryl, and, indeed, from -tourmaline also. - -Judged by the criterion of price, topaz is not in the first rank of -precious stones. Stones of good colour and free from flaws are now, -however, scarce. Pale stones are worth very little, possibly less than -4s. a carat, but the price rapidly advances with increase in colour, -reaching 20s. for yellow, 80s. for pink and blue stones. Since topazes -are procurable in all sizes customary in jewellery, the rates vary but -slightly, if at all, with the size. - -Topaz occurs principally in pegmatite dykes and in cavities in granite, -and is interesting to petrologists as a conspicuous instance of the -result of the action of hot acid vapours upon rocks rich in aluminium -silicates. Magnificent crystals have come from the extensive mining -district which stretches along the eastern flank of the Ural Mountains, -and from the important mining region surrounding Nertschinsk, in the -Government of Transbaikal, Siberia. Fine green and blue stones have -been found at Alabashka, near Ekaterinburg, in the Government of Perm, -and at Miask in the Ilmen Mountains, in the Government of Orenburg. -Topazes of the rare reddish hue have been picked out from the gold -washings of the Sanarka River, Troitsk, also in the Government of -Orenburg. Splendid pale-brown stones have issued from the Urulga River, -near Nertschinsk, and good crystals have come from the Adun-Tschilon -Mountains. Kamchatka has produced yellow, blue, and green stones. In -the British Isles, beautiful sky-blue, waterworn crystals have been -found at Cairngorm, Banffshire, in Scotland, and colourless stones in -the Mourne Mountains, Ireland, and at St. Michael’s Mount, Cornwall. -Most of the topazes used in jewellery of the present day come from -either Brazil or Ceylon. Ouro Preto, Villa Rica, and Minas Novas, in -the State of Minas Geraes, are the principal localities in Brazil. -Numerous stones, often waterworn, brilliant and colourless or tinted -lovely shades of blue and wine-yellow, occur there; reddish stones also -have been found at Ouro Preto. Ceylon furnishes a profusion of yellow, -light-green, and colourless, waterworn pebbles. The colourless stones -found there are incorrectly termed by the natives ‘water-sapphire,’ and -the light-green stones are sold with beryl as aquamarines; the stones -locally known as ‘king topaz’ are really yellow corundum (cf. p. 181). -Colourless crystals, sometimes with a faint tinge of colour, have -been discovered in many parts of the world, such as Ramona, San Diego -County, California, and Pike’s Peak, Colorado, in the United States, -San Luis Potosi in Mexico, and Omi and Otami-yama in Japan. - - - - - CHAPTER XXII - - SPINEL - - (_Balas-Ruby_, _Rubicelle_) - - -Spinel labours under the serious disadvantage of being overshadowed at -almost all points by its opulent and more famous cousins, sapphire and -ruby, and is not so well known as it deserves to be. The only variety -which is valued as a gem is the rose-tinted stone called balas-ruby -(Plate XXVII, Fig. 3), which is very similar to the true ruby in -appearance; they are probably often confused, especially since they -are found in intimate association in nature. Spinels of other colours -are not very attractive to the eye, and are not likely to be in much -demand. Blue spinel (Plate XXVII, Fig. 4) is far from common, but the -shade is inclined to steely-blue, and is much inferior to the superb -tint of the true sapphire. Spinel is very hard and eminently suitable -for a ring-stone, but is seldom large and transparent enough for larger -articles of jewellery. - -Spinel is an aluminate of magnesium corresponding to the formula -MgAl_{2}O_{4}, and therefore is closely akin to corundum, alumina, and -chrysoberyl, aluminate of beryllium. The composition may, however, -vary considerably owing to the isomorphous replacement of one element -by another; in particular, ferrous oxide or manganese oxide often -takes the place of some magnesia, and ferric oxide or chromic oxide is -found instead of part of the alumina. When pure, spinel is devoid of -colour, but such stones are exceedingly rare. No doubt chromic oxide is -responsible for the rose-red hue of balas-ruby, and also, when tempered -by ferric oxide, for the orange tint of rubicelle, and manganese is -probably the cause of the peculiar violet colour of almandine-spinel. -It is scarcely possible to define all the shades between blue and red -that may be assumed by spinel. Stones which are rich in iron are known -as pleonaste or ceylonite; they are quite opaque, but are sometimes -used for ornamental wear. - -The name of the species comes from a diminutive form of σπῖνος, a -spark, and refers to the fiery red colour of the most valued kind -of spinel. It may be noted that the Latin equivalent of the word, -_carbunculus_, has been applied to the crimson garnet when cut -_en cabochon_. Balas is derived from _Balascia_, the old name for -Badakshan, the district from which the finest stones were brought in -mediæval times. - -[Illustration: FIGS. 73, 74.—Spinel Crystals.] - -Spinel, like diamond, belongs to the cubic system of crystalline -symmetry, and occurs in beautiful octahedra, or in flat -triangular-shaped plates (Figs. 73, 74) the girdles of which are -cleft at each corner, these plates being really twinned octahedra. -The refraction is, of course, single, and there is therefore no -double refraction or dichroism; this test furnishes the simplest -way of discriminating between the balas and the true ruby. Owing to -isomorphous replacement the value of the refractive index may lie -anywhere between 1·716 and 1·736. The lower values, about 1·720, -correspond to the most transparent red and blue stones; the deep -violet stones have values above 1·730. Spinel possesses little -colour-dispersion, or ‘fire.’ In the same way the values of the -specific gravity, even of the transparent stones, vary between 3·5 -and 3·7, but the opaque ceylonite has values as high as 4·1. Spinel -is slightly softer than sapphire and ruby, and has the symbol 8 on -Mohs’s scale, and it is scarcely inferior in lustre to these stones. -Spinel is easily separated from garnet of similar colour by its lower -refractivity. Spinels run from 10s. to £5 a carat, depending on their -colour and quality, and exceptional stones command a higher rate. - -Spinel always occurs in close association with corundum. The balas -and the true ruby are mixed together in the limestones of Burma and -Siam. Curiously enough, the spinel despite its lower hardness is found -in the river gravels in perfect crystals, whereas the rubies are -generally waterworn. Fine violet and blue spinels occur in the prolific -gem-gravels of Ceylon. A large waterworn octahedron and a rough mass, -both of a fine red colour, are exhibited in the Mineral Gallery of the -British Museum (Natural History), and a beautiful faceted blue stone is -shown close by. - -The enormous red stone, oval in shape, which is set in front of the -English crown, is not a ruby, as it was formerly believed to be, -but a spinel. It was given to the gallant Black Prince by Pedro the -Cruel after the battle of Najera in 1367, and was subsequently worn -by Henry V upon his helmet at the battle of Agincourt. As usual with -Indian-fashioned stones it is pierced through the middle, but the hole -is now hidden by a small stone of similar colour. - -The British Regalia also contains the famous stone called the Timur -Ruby or Khiraj-i-Alam (Tribute of the World), which weighs just over -352 carats, and is the largest spinel-ruby known. It is uncut, but -polished. Its history goes back to 1398, when it was captured by the -Amir Timur at Delhi. On the wane of the Tartar empire the stone became -the property of the Shahs of Persia, until it was given by Abbas I to -his friend and ally, the Mogul Emperor, Jehangir. It remained at Delhi -until, on the sack of that city by Nadir Shah in 1739, it, together -with immense booty, including the Koh-i-nor, fell into the hands of -the conqueror. Like the great diamond, it eventually came into the -possession of Runjit Singh at Lahore, and on the annexation of the -Punjab in 1850 passed to the East India Company. It was shown at the -Great Exhibition of 1851, and afterwards presented to Queen Victoria. - -Mention has been made above (p. 121) of the blue spinel which is -manufactured in imitation of the true sapphire. The artificial stone is -quite different in tint from the blue spinel found in nature. - - - - - CHAPTER XXIII - - GARNET - - -The important group of minerals which are known under the general name -of garnet provides an apt illustration of the fact that rarity is an -essential condition if a stone is to be accounted precious. Owing to -the large quantity of Bohemian garnets, of a not very attractive shade -of yellowish red, that have been literally poured upon the market -during the past half-century the species has become associated with -cheap and often ineffective jewellery, and has acquired a stigma which -completely prevents its attaining any popularity with those professing -a nice taste in gem-stones. It must not, however, be supposed that -garnet has entirely disappeared from high-class jewellery although the -name may not readily be found in a jeweller’s catalogue. Those whose -business it is to sell gem-stones are fully alive to the importance of -a name, and, as has already been remarked (p. 109), they have been fain -to meet the prejudices of their customers by offering garnets under -such misleading guises as ‘Cape-ruby,’ ‘Uralian emerald,’ or ‘olivine.’ - -Garnets may, moreover, figure under another name quite unintentionally. -Probably many a fine stone masquerades as a true ruby; the -impossibility of distinguishing these two species in certain cases by -eye alone is perhaps not widely recognized. An instructive instance -came under the writer’s notice a few years ago. A lady one day had -the misfortune to fracture one of the stones in a ruby ring that had -been in the possession of her family for upwards of a century, and -was originally purchased of a leading firm of jewellers in London. -She took the ring to her jeweller, and asked him to have the stone -replaced by another ruby. A day or two later he sent word that it was -scarcely worth while to put a ruby in because the stones in the ring -were paste. Naturally distressed at such an opinion of a ring which had -always been held in great esteem by her family, the lady consulted a -friend, who suggested showing it to the writer. A glance was sufficient -to prove that if the ring had been in use so long the stones could not -possibly be paste on account of the excellent state of their polish, -but a test with the refractometer showed that the stones were really -almandine-garnets, which so often closely resemble the true ruby in -appearance. Beautiful as the stones were, the ring was probably not -worth one-tenth what the value would have been had the stones been -rubies. - -To the student of mineralogy garnet is for many reasons of peculiar -interest. It affords an excellent illustration of the facility which -certain elements possess for replacing one another without any great -disturbance of the crystalline form. Despite their apparent complexity -in composition all garnets conform to the same type of formula: lime, -magnesia, and ferrous and manganese oxides, and again alumina and -ferric and chromic oxides may replace each other in any proportion, -iron being present in two states of oxidation, and it would be rare -to find a stone which agrees in composition exactly with any of the -different varieties of garnet given below. - -[Illustration: FIGS. 75, 76.—Garnet Crystals.] - -Garnet belongs to the cubic system of crystalline symmetry. -Its crystals are commonly of two kinds, both of which are very -characteristic, the regular dodecahedron, _i.e._ twelve-faced figure -(Fig. 75), and the tetrakis-octahedron or three-faced octahedron (Fig. -76); the latter crystals are, especially when weather- or water-worn, -almost spherical in shape. Closer and more refined observations have -shown that garnet is seldom homogeneous, being usually composed of -several distinct individuals of a lower order of symmetry. Although -singly refractive as far as can be determined with the refractometer or -by deviation through a prism, yet when examined under the polarizing -microscope, garnets display invariably a small amount of local double -refraction. The transition from light to darkness is, however, not -sharp as in normal cases, but is prolonged into a kind of twilight. -In hardness, garnet is on the whole about the same as quartz, but -varies slightly; hessonite and andradite are a little softer, pyrope, -spessartite, and almandine are a little harder, while uvarovite is -almost the same. All the varieties except uvarovite are fusible when -heated before the blowpipe, and small fragments melt sufficiently on -the surface in the ordinary bunsen flame to adhere to the platinum -wire holding them. This test is very useful for separating rough red -garnets, pyrope or almandine, from red spinels or zircons of very -similar appearance. Far greater variation occurs in the other physical -characters. The specific gravity may have any value between 3·55 and -4·20, and the refractive index ranges between 1·740 and 1·890. Both the -specific gravity and the refractive index increase on the whole with -the percentage amount of iron. - -Garnet is a prominent constituent of many kinds of rocks, but the -material most suitable for gem purposes occurs chiefly in crystalline -schists or metamorphic limestones. Pyrope and demantoid are furnished -by peridotites and the serpentines resulting from them; almandine and -spessartite come mostly from granites. - -The name of the species is derived from the Latin _granatus_, -seed-like, and is suggested by the appearance of the spherical crystals -when embedded in their pudding-like matrix. - -The varieties most adapted to jewellery are the fiery-red pyrope and -the crimson and columbine-red almandine; the closer they approach the -ruddy hue of ruby the better they are appreciated. Hessonite was at one -time in some demand, but it inclines too much to the yellowish shade -of red and possesses too little perfection of transparency to accord -with the taste of the present day. Demantoid provides beautiful, pale -and dark emerald-green stones, of brilliant lustre and high dispersion, -which are admirably adapted for use in pendants or necklaces; on -account of their comparative softness it would be unwise to risk them -in rings. In many stones the colour takes a yellowish shade, which -is less in demand. Uvarovite also occurs in attractive emerald-green -stones, but unfortunately none as yet have been found large enough for -cutting. A few truly magnificent spessartites are known—one, a splendid -example, weighing 6¾ carats, being in the possession of Sir Arthur -Church; but the species is far too seldom transparent to come into -general use. The price varies per carat from 2s. for common garnet to -10s. for stones most akin to ruby in colour, and exceptional demantoids -may realize even as much as £10 a carat. The old style of cutting was -almost invariably rounded or _en cabochon_, but at the present day the -brilliant-cut front and the step-cut back is most commonly adopted. - -The several varieties will now be considered in detail. - - - (_a_) HESSONITE - - (_Grossular_, _Cinnamon-Stone_, _Hyacinth_, _Jacinth_) - -This variety, strictly a calcium-aluminium garnet corresponding to -the formula Ca_{3}Al_{2}(SiO_{4})_{3}, but generally containing some -ferric oxide and therefore tending towards andradite, is called by -several different names. In science it is usually termed grossular, a -word derived from _grossularia_, the botanical name for gooseberry, in -allusion to the colour and appearance of many crystals, or hessonite, -and less correctly essonite, words derived from the Greek ἥσσων -in reference to the inferior hardness of these stones as compared -with zircon of similar colour; in jewellery it is better known as -cinnamon-stone, if a golden-yellow in colour, or hyacinth or jacinth. -The last word, which is indiscriminately used for hessonite and yellow -zircon, but should more properly be applied to the latter, is derived -from an old Indian word (cf. p. 229); jewellers, however, retain it for -the garnet. - -Only the yellow and orange shades of hessonite (Plate XXIX, Fig. 5) -are used for jewellery. Neither the brownish-green kind, to which the -term grossular may properly be applied, nor the rose-red is transparent -enough to serve as a gem-stone. Hessonite may mostly be recognized, -even when cut, by the curiously granular nature of its structure, just -as if it were composed of tiny grains imperfectly fused together; this -appearance, which is very characteristic, may readily be perceived if -the interior of the stone be viewed through a lens of moderate power. - -The specific gravity varies from 3·55 to 3·66, and the refractive index -from 1·742 to 1·748. The hardness is on the whole slightly below that -of quartz. When heated before a blowpipe it easily fuses to a greenish -glass. - -The most suitable material is found in some profusion in the -gem-gravels of Ceylon, in which it is mixed up with zircon of an almost -identical appearance; both are called hyacinth. Hessonites from other -localities, although attractive as museum specimens, are not large and -clear enough for cutting purposes. Switzerland at one time supplied -good stones, but the supply has long been exhausted. - - - (_b_) PYROPE - - (‘_Cape-Ruby_’) - -Often quite ruby-red in colour (Plate XXIX, Fig. 6), this variety -is probably the most popular of the garnets. It is strictly -a magnesium-aluminium garnet corresponding to the formula -Mg_{3}Al_{2}(SiO_{4})_{3}, but usually contains some ferrous oxide -and thus approaches almandine. Both are included among the precious -garnets. Its name is derived from πυρωπός, fire-like, in obvious -allusion to its characteristic colour. - -Although at its best pyrope closely resembles ruby, its appearance is -often marred by a tinge of yellow which decidedly detracts from its -value. Pyrope generally passes as a variety of ruby, and under such -names as ‘Cape-ruby,’ ‘Arizona-ruby,’ depending on the origin of the -stones, commands a brisk sale. The specific gravity varies upwards -from 3·70, depending upon the percentage amount of iron present, and -similarly the refractive index varies upwards from 1·740; in the higher -values pyrope merges into almandine. Its hardness is slightly greater -than that of quartz, and may be expressed on Mohs’s scale by the symbol -7¼. - -An enormous quantity of small red stones, mostly with a slight tinge of -yellow, have been brought to light at Teplitz, Aussig, and other spots -in the Bohemian Mittelgebirge, and a considerable industry in cutting -and marting them has grown up at Bilin. Fine ruby-red stones accompany -diamond in the ‘blue ground’ of the mines at Kimberley and also at the -Premier mine in the Transvaal. Similar stones are also found in Arizona -and Colorado in the United States, and in Australia, Rhodesia, and -elsewhere. - -Although commonly quite small in size, pyrope has occasionally attained -to considerable size. According to De Boodt the Kaiser Rudolph II had -one in his possession valued at 45,000 thalers (about £6750). The -Imperial Treasury at Vienna contains a stone as large as a hen’s egg. -Another about the size of a pigeon’s egg is in the famous Green Vaults -at Dresden, and the King of Saxony has one, weighing 468½ carats, set -in an Order of the Golden Fleece. - - - (_c_) RHODOLITE - -This charming pale-violet variety was found at Cowee Creek and at -Mason’s Branch, Macon County, North Carolina, U.S.A., but in too -limited amount to assume the position in jewellery it might otherwise -have expected. In composition it lies between pyrope and almandine, -and may be supposed to contain a proportion of two molecules of the -former to one of the latter. Its specific gravity is 3·84, refractive -index 1·760, and hardness 7¼. It exhibits in the spectroscope the -absorption-bands characteristic of almandine. - - - (_d_) ALMANDINE - - (_Carbuncle_) - -This variety is iron-aluminium garnet corresponding to the formula -Fe_{3}Al_{2}(SiO_{4})_{3}, but the composition is very variable. In -colour it is deep crimson and violet or columbine-red (Plate XXIX, Fig. -8), but with increasing percentage amount of ferric oxide it becomes -brown and black, and opaque, and quite unsuitable for jewellery. The -name of the variety is a corruption of Alabanda in Asia Minor, where -in Pliny’s time the best red stones were cut. Almandine is sometimes -known as Syriam, or incorrectly Syrian garnet, because at Syriam, once -the capital of the ancient kingdom of Pegu, which now forms part of -Lower Burma, such stones were cut and sold. Crimson stones, cut in the -familiar _en cabochon_ form and known as carbuncles, were extensively -employed for enriching metalwork, and a half-century or so ago were -very popular for ornamental wear, but their day has long since gone. -Such glowing stones are aptly described by their name, which is derived -from the Latin _carbunculus_, a little spark. In Pliny’s time, however, -the term was used indiscriminately for all red stones. It has already -been remarked that the word spinel has a similar significance. - -The specific gravity varies from 3·90 for transparent stones to 4·20 -for the densest black stones, and the refractive index may be as high -as 1·810. Almandine is one of the hardest of the garnets, and is -represented by the symbol 7½ on Mohs’s scale. The most interesting and -curious feature of almandine lies in the remarkable and characteristic -absorption-spectrum revealed when the transmitted light is examined -with a spectroscope (p. 61). The phenomenon is displayed most vividly -by the violet stones, and is, indeed, the cause of their peculiar -colour. - -Although a common mineral, almandine of a quality fitted for jewellery -occurs in comparatively few localities. It is found in Ceylon, but -not so plentifully as hessonite. Good stones are mined in various -parts of India, and are nearly all cut at Delhi or Jaipur. Brazil -supplies good material, especially in the Minas Novas district of -Minas Geraes, where it accompanies topaz, and Uruguay also furnishes -serviceable stones. Almandine is found in Australia, and in many parts -of the United States. Recently small stones of good colour have been -discovered at Luisenfelde in German East Africa. - - - (_e_) SPESSARTITE - -Properly a manganese-aluminium garnet corresponding to the formula -Mn_{3}Al_{2}(SiO_{4})_{3}, this variety generally contains iron in -both states of oxidation. If only transparent and large enough its -aurora-red colour would render it most acceptable in jewellery. Two -splendid stones have, indeed, been found in Ceylon (p. 211), and good -stones rather resembling hessonites have been quarried at Amelia -Court House in Virginia, and others have come from Nevada; otherwise, -spessartite is unknown as a gem-stone. - -The specific gravity ranges from 4·0 to 4·3, and the refractive index -is about 1·81, both characters being high; the hardness is slightly -greater than that of quartz. - - - (_f_) ANDRADITE - - (_Demantoid_, _Topazolite_, ‘_Olivine_’) - -Andradite is strictly a calcium-iron garnet corresponding to the -formula Ca_{3}Fe_{2}(SiO_{4})_{3}, but as usual the composition varies -considerably. It is named after d’Andrada, a Portuguese mineralogist, -who made a study of garnet more than a century ago. - -Once contemptuously styled common garnet, andradite suddenly sprang -into the rank of precious stones upon the discovery some thirty -years ago of the brilliant, green stones (Plate XXIX, Fig. 7) in the -serpentinous rock beside the Bobrovka stream, a tributary of the -Tschussowaja River, in the Sissersk district on the western side of -the Ural Mountains. The shade of green varies from olive through -pistachio to a pale emerald, and is probably due to chromic oxide. Its -brilliant lustre, almost challenging that of diamond, and its enormous -colour-dispersion, in which respect it actually transcends diamond, -raise it to a unique position among coloured stones. Unfortunately -its comparative softness limits it to such articles of jewellery as -pendants and necklaces, where it is not likely to be rubbed. When first -found it was supposed to be true emerald, which does actually occur -near Ekaterinburg, and was termed ‘Uralian emerald.’ When analysis -revealed its true nature, it received from science the slightly -inharmonious name of demantoid in compliment to its adamantine lustre. -Jewellers, however, prefer to designate it ‘olivine,’ not very happily, -because the stones usually cut are not olive-green and the name is -already in extensive use in science for a totally distinct species (p. -225); they recognized the hopelessness of endeavouring to find a market -for them as garnets. The yellow kind of andradite known as topazolite -would be an excellent gem-stone if only it were found large and -transparent enough. Ordinary andradite is brown or black, and opaque; -it has occasionally been used for mourning jewellery. - -The specific gravity varies from 3·8 to 3·9, being about 3·85 for -demantoid, which has a high refractive index, varying from 1·880 to -1·890, and may with advantage be cut in the brilliant form. It is the -softest of the garnets, being only 6½ on Mohs’s scale. - - - (_g_) UVAROVITE - -This variety, which is altogether unknown in jewellery, is -a calcium-iron garnet corresponding mainly to the formula -Ca_{3}Cr_{2}(SiO_{4})_{3}, but with some alumina always present, and -was named after a Russian minister. It has an attractive green colour, -and is, moreover, hard, being about on Mohs’s scale, but it has never -yet come to light of a size suitable for cutting. The specific gravity -is low, varying from 3·41 to 3·52. Unlike the kindred varieties it -cannot be fused by heating before an ordinary blowpipe. - - - - - CHAPTER XXIV - - TOURMALINE - - (_Rubellite_) - - -Tourmaline is unsurpassed even by corundum in variety of hue, and -it has during recent years rapidly advanced in public favour, -mainly owing to the prodigal profusion in which nature has formed -it in that favoured State, California, the garden of the west. Its -comparative softness militates against its use in rings, but its -gorgeous coloration renders it admirably fitted for service in any -article of jewellery, such as a brooch or a pendant, in which a large -central stone is required. Like all coloured stones it is generally -brilliant-cut in front and step-cut at the back, but occasionally it is -sufficiently fibrous in structure to display, when cut _en cabochon_, -pronounced chatoyancy. - -The composition of this complex species has long been a vexed -question among mineralogists, but considerable light was -recently thrown on the subject by Schaller, who showed that all -varieties of tourmaline may be referred to a formula of the type -12SiO_{2}.3B_{2}O_{3}.(9-_x_)[(Al,Fe)_{2}O_{3}].3_x_[(Fe,Mn,Ca, -Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O. The ratios of boric oxide, -silica, and water are nearly constant in all analyses, but great -variation is possible in the proportions of the other constituents. -Having regard to this complexity, it is not surprising to find that -the range in colour is so great. Colourless stones, to which the name -achroite is sometimes given, were at one time exceedingly rare, but -they are now found in greater number in California. Stones which are -most suited to jewellery purposes are comparatively free from iron, and -apparently owe their wonderful tints to the alkaline earths; lithia, -for instance, is responsible for the beautiful tint of the highly -prized rubellite, and magnesia, no doubt, for the colour of the brown -stones of various tints. Tourmaline rich in iron is black and almost -opaque. It is a striking peculiarity of the species that the crystals -are rarely uniform in colour throughout, the boundaries between the -differently coloured portions being sharp and abrupt, and the tints -remarkably in contrast. Sometimes the sections are separated by planes -at right angles to the length of the crystal, and sometimes they are -zonal, bounded by cylindrical surfaces running parallel to the same -length. In the latter case a section perpendicular to the length shows -zones of at least three contrasting tints. In the Brazilian stones the -core is generally red, bounded by white, with green on the exterior, -while the reverse is the case in the Californian stones, the core being -green or yellow, bounded by white, with red on the exterior. Tourmaline -may, indeed, be found of almost every imaginable tint, except, -perhaps, the emerald green and the royal sapphire-blue. The principal -varieties are rose-red and pink (rubellite) (Plate XXVII, Fig. 1), -green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian -sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2), -honey-yellow (Ceylonese peridot), violet-red (siberite), and brown -(Plate XXVII, Fig. 8). The black, opaque stones are termed schorl. - -The name of the species is derived from the Ceylonese word, _turamali_, -and was first employed when a parcel of gem-stones was brought to -Amsterdam from Ceylon in 1703; in Ceylon, however, the term is applied -by native jewellers to the yellow zircon commonly found in the island. -Schorl, the derivation of which is unknown, is the ancient name for the -species, and is still used in that sense by miners, but it has been -restricted by science to the black variety. The ‘Brazilian emerald’ -was introduced into Europe in the seventeenth century and was not -favourably received, possibly because the stones were too dark in -colour and were not properly cut; that they should have been confused -with the true emerald is eloquent testimony to the extreme ignorance -of the characters of gem-stones prevalent in those dark ages. Achroite -comes from the Greek, ἄχροος, without colour. - -[Illustration: FIG. 77.—Tourmaline Crystal.] - -To the crystallographer tourmaline is one of the most interesting of -minerals. If the crystals, which are usually prismatic in form, are -doubly terminated, the development is so obviously different at the -two ends (Fig. 77) as to indicate that directional character in the -molecular arrangement, termed the polarity, which is borne out by other -physical properties. Tourmaline is remarkably dichroic, A brown stone, -except in very thin sections, is practically opaque to the ordinary -ray, and consequently a section cut parallel to the crystallographic -axis, _i.e._ to the length of a crystal prismatically developed, -transmits only the extraordinary ray. Such sections were in use for -yielding plane-polarized light before Nicol devised the calcite prism -known by his name (cf. p. 44). It is evident that tourmaline, unless -very light in tint, must be cut with the table facet parallel to that -axis, because otherwise the stone will appear dark and lifeless. The -values of the extraordinary and ordinary refractive indices range -between 1·614 and 1·638, and 1·633 and 1·669 respectively; the double -refraction, therefore, is fairly large, amounting to 0·025, and, -since the ordinary exceeds the extraordinary ray, its character is -negative. The specific gravity varies from 3·0 to 3·2. The lower values -in both characters correspond to the lighter coloured stones used in -jewellery; the black stones, as might be expected from their relative -richness in iron, are the densest. The hardness is only about the same -as that of quartz, or perhaps a little greater, varying from 7 to 7½. -It will be noticed that the range of refractivity overlaps that of -topaz (_q.v._) but the latter has a much smaller double refraction, -and may thus be distinguished (p. 29). Unmounted stones are still more -easily distinguished, because tourmaline floats in methylene iodide, -while topaz sinks. The pyro-electric phenomenon (cf. p. 82) for which -tourmaline is remarkable, although of little value as a test in the -case of a cut stone, is of great scientific interest, because it is -strong evidence of the peculiar crystalline symmetry pertaining to its -molecular arrangement. Tourmalines range in price from 5s. to 20s. a -carat according to their colour and quality, but exceptional stones may -command a higher rate. - -Tourmaline is usually found in the pegmatite dykes of granites, but -it also occurs in schists and in crystalline limestones. Rubellite -is generally associated with the lithia mica, lepidolite; the groups -of delicate pink rubellite bespangling a background of greyish -white lepidolite are among the most beautiful of museum specimens. -Magnificent crystals of pink, blue, and green tourmaline have been -found in the neighbourhood of Ekaterinburg, principally at Mursinka, in -the Urals, Russia, and fine rubellite has come from the Urulga River, -and other spots near Nertschinsk, Transbaikal, Asiatic Russia. Elba -produces pink, yellowish, and green stones, frequently particoloured; -sometimes the crystals are blackened at the top, and are then known -locally as ‘nigger-heads.’ Ceylon supplies small yellow stones—the -original tourmaline—which are confused with the zircon of a similar -colour, and rubellite accompanies the ruby at Ava, Burma. Beautiful -crystals, green and red, often diversely coloured, come from various -parts, such as Minas Novas and Arassuahy, of the State of Minas Geraes, -Brazil. Suitable gem material has been found in numerous parts of -the United States. Paris and Hebron in Maine have produced gorgeous -pink and green crystals, and Auburn in the same state has supplied -deep-blue, green, and lilac stones. Fine crystals, mostly green, but -also pink and particoloured, occur in an albite quarry near the Conn -River at Haddam Neck, Connecticut. All former localities have, however, -been surpassed by the extraordinary abundance of superb green, and -especially pink, crystals at Pala and Mesa Grande in San Diego County, -California. As elsewhere, many-hued stones are common. The latter -locality supplies the more perfectly transparent crystals. Kunz states -that two remarkable rubellite crystals were found there, one being 45 -mm. in length and 42 mm. in diameter, and the other 56 mm. in length -and 24 mm. in diameter. Madagascar, which has proved of recent years -to be rich in gem-stones, supplies green, yellow, and red stones, both -uniformly tinted and particoloured, which in beauty, though perhaps not -in size, bear comparison with any found elsewhere. - - - - - CHAPTER XXV - - PERIDOT - - -The beautiful bottle-green stone, which from its delicate tint has -earned from appreciative admirers the poetical _sobriquet_ of the -evening emerald, and which has during recent years crept into popular -favour and now graces much of the more artistic jewellery, is named -as a gem-stone peridot—a word long in use among French jewellers, the -origin and meaning of which has been forgotten—but is known to science -either as olivine, on account of the olive-green colour sometimes -characterizing it, or as chrysolite. It is of interest to note that the -last word, derived from χρυσός, golden, and λίθος, stone, was in use at -the time of Pliny, but was employed for topaz and other yellow stones, -while his topaz, curiously enough, designated the modern peridot (cf. -p. 199), an inversion that has occurred in other words. The true -olivine must not be confused with the jewellers’ ‘olivine,’ which is a -green garnet from the Ural Mountains (p. 217). Peridot is comparatively -soft, the hardness varying from 6½ to 7 on Mohs’s scale, and is -suitable only for articles which are not likely to be scratched; the -polish of a peridot worn in a ring would soon deteriorate. The choicest -stones are in colour a lovely bottle-green (Plate XXIX, Fig. 2) of -various depths; the olive-green stones (Plate XXIX, Fig. 3) cannot -compare with their sisters in attractiveness. The step form of cutting -is considered the best for peridot, but it is sometimes cut round or -oval in shape, with brilliant-cut fronts. - -Peridot is a silicate of magnesium and iron, corresponding to the -formula (Mg,Fe)_{2}SiO_{4}, ferrous iron, therefore, replacing -magnesia. To the ferrous iron it is indebted for its colour, the -pure magnesium silicate being almost colourless, and the olive tint -arises from the oxidation of the iron. The latitude in the composition -resulting from this replacement is evinced in the considerable range -that has been observed in the physical characters, but the crystalline -symmetry persists unaltered; the lower values correspond to the stones -that are usually met with as gems. Peridot belongs to the orthorhombic -system of crystalline symmetry, and the crystals, which display a -large number of faces, are prismatic in form and generally somewhat -flattened. The stones, however, that come into the market for cutting -as gems are rarely unbroken. The dichroism is rather faint, one of the -twin colours being slightly more yellowish than the other, but it is -more pronounced in the olive-tinted stones. The values of the least -and greatest of the principal indices of refraction vary greatly, -from 1·650 and 1·683 to 1·668 and 1·701, but the double refraction, -amounting to 0·033, remains unaffected. Peridot, though surpassed by -sphene in extent of double refraction, easily excels all the ordinary -gem-stones in this respect, and this character is readily recognizable -in a cut stone by the apparent doubling of the opposite edges when -viewed through the table facet (cf. p. 41). An equally large -variation occurs in the specific gravity, namely, from 3·3 to 3·5. - -[Illustration: _PLATE XXVII_ - - 1. RUBELLITE - 2. TOURMALINE - 3. BALAS-RUBY - 4. BLUE SPINEL - 5. QUARTZ - 6. WHITE OPAL - 7. AMETHYST - 8. TOURMALINE - 9. BLACK OPAL - 10. FIRE OPAL - 11. ALEXANDRITE (_By daylight_) - 12. CHRYSOBERYL - 13. ALEXANDRITE (_By artificial light_) - -GEM-STONES] - -Peridots of deep bottle-green hue command moderate prices at the -present day, about 30s. a carat being asked for large stones; the paler -tinted stones run down to a few shillings a carat. The rate per carat -may be very much larger for stones of exceptional size and quality. - -Olivine, to use the ordinary mineralogical term, is a common and -important constituent of certain kinds of igneous rocks, and it is -also found in those strange bodies, meteorites, which come to us -from outer cosmical space. Except in basaltic lavas, it occurs in -grains and rarely in well-shaped crystals. Stones that are large and -transparent enough for cutting purposes come almost entirely from the -island Zebirget or St. John situated on the west coast of the Red Sea, -opposite to the port of Berenice. This island belongs to the Khedive of -Egypt, and is at present leased to a French syndicate. It is believed -to be the same as the mysterious island which produced the ‘topaz’ -of Pliny’s time. Magnificent stones have been discovered here, rich -green in colour, and 20 to 30, and occasionally as much as 80, carats -in weight when cut; a rough mass attained to the large weight of 190 -carats. Pretty, light-green stones are supplied by Queensland, and -peridots of a less pleasing dark-yellowish shade of green, and without -any sign of crystal form, have during recent years come from North -America. Stones rather similar to those from Queensland have latterly -been found in the Bernardino Valley in Upper Burma, not far from the -ruby mines. - - - - - CHAPTER XXVI - - ZIRCON - - (_Jargoon_, _Hyacinth_, _Jacinth_) - - -Zircon, which, if known at all in jewellery, is called by its variety -names, jargoon and hyacinth or jacinth, is a species that deserves -greater recognition than it receives. The colourless stones rival -even diamond in splendour of brilliance and display of ‘fire’; the -leaf-green stones (Plate XXIX, Fig. 13) possess a restful beauty -that commends itself; the deep-red stones (Plate XXIX, Fig. 14), if -somewhat sombre, have a certain grandeur; and no other species produces -such magnificent stones of golden-yellow hue (Plate XXIX, Fig. 12). -Zircon is well known in Ceylon, which supplies the world with the -finest specimens, and is highly appreciated by the inhabitants of that -sunny isle, but it scarcely finds a place in jewellery elsewhere. The -colourless stones are cut as brilliants, but brilliant-cut fronts with -step-cut backs is the usual style adopted for the coloured stones. - -Zircon is a silicate of zirconium corresponding to the formula -ZrSiO_{4}, but uranium and the rare earths are generally present in -small quantities. The aurora-red variety is known as hyacinth or -jacinth, and the term jargoon is applied to the other transparent -varieties, and especially to the yellow stones. The most attractive -colours shown by zircon are leaf-green, golden-yellow, and deep red. -Other common colours are brown, greenish, and sky-blue. Colourless -stones are not found in nature, but result from the application of heat -to the yellow and brown stones. - -The name of the species is ancient, and comes from the Arabic _zarqūn_, -vermilion, or the Persian _zarqūn_, gold-coloured. From the same -source in all probability is derived the word jargoon through the -French _jargon_ and the Italian _giacone_. Hyacinth (cf. p. 211) is -transliterated from the Greek ὑάκινθος, itself adapted from an old -Indian word; it is in no way connected with the flower of the same -name. The last word has seen some changes of meaning. In Pliny’s time -yellow zircons were indiscriminately classified with other yellow -stones as chrysolite. His hyacinth was used for the sapphire of the -present day, but was subsequently applied to any transparent corundum. -Upon the introduction of the terms, sapphire and ruby, for the blue and -the red corundum hyacinth became restricted to the other varieties, of -which the yellow was the commonest. In the darkness of the Middle Ages -it was loosely employed for all yellow stones emanating from India, -and was finally, with increasing discernment in the characters of -gem-stones, assigned to the yellow zircon, since it was the commonest -yellow stone from India. - -[Illustration: FIG. 78.—Zircon Crystal.] - -Considered from the scientific point of view, zircon is by far the most -interesting and the most remarkable of the gem-stones. The problem -presented by its characters and constitution is one that still awaits -a satisfactory solution. Certain zircons, which are found as rolled -pebbles in Ceylon and never show any trace of crystalline faces, have -very nearly single refraction, and the values of the refractive index -vary from 1·790 to 1·840, and the specific gravity is about 4·00 to -4·14, and the hardness is slightly greater than that of quartz, being -about 7¼. On the other hand, such stones as the red zircons from -Expailly have remarkably different properties. They show crystalline -faces with tetragonal symmetry, the faces present being four prismatic -faces mutually intersecting at right angles and four inclined faces at -each end (Fig. 78). They have large double refraction, varying from -0·044 to 0·062, which is readily discerned in a cut stone (cf. p. -41), and the refractive indices are high, the ordinary index varying -from 1·923 to 1·931 and the extraordinary from 1·967 to 1·993. Since -the ordinary is less than the extraordinary index the sign of the -double refraction is positive. The specific gravity likewise is much -higher, varying from 4·67 to 4·71. The second type, therefore, sinks -in molten silver-thallium nitrate, whereas the first type floats. The -second type is also slightly harder, being about 7½ on Mohs’s scale. -By heating either of these types the physical characters are not much -altered, except that the colour is weakened or entirely driven off and -some change takes place in the double refraction. But between these -two types may be found zircons upon which the effect of heating is -striking. They seem to contract in size so that the specific gravity -increases as much as three units in the first place of decimals, and a -corresponding increase takes place in the refractive indices, and in -the amount of double refraction. The cause of these changes remains a -matter of speculation. Evidently a third type of zircon exists which -is capable of most intimate association with either of the other -types, and which is very susceptible to the effect of heat. It may be -noted that stones of the intermediate type are usually characterized -by a banded or zonal structure suggesting a want of homogeneity. The -theory has been advanced that zircon contains an unknown element which -has not yet been separated from zirconium. Zircon of the first type -favours green, sky-blue, and golden-yellow colours; honey-yellow, light -green, blue, and red colours characterize the second type; and the -intermediate stones are mostly yellowish green, cloudy blue, and green. - -It is another peculiarity of zircon that it sometimes shows in the -spectroscope absorption bands (p. 61), which were observed in 1866 by -Church. Many zircons do not exhibit the bands at all, and others only -display the two prominent bands in the red end of the spectrum. - -Of all the gem-stones zircon alone approaches diamond in brilliance of -lustre, and it also possesses considerable ‘fire’; it can, of course, -be readily distinguished by its inferior hardness, but a judgment based -merely on inspection by eye might easily be erroneous. - -According to Church, who has made a lifelong study of zircon, the green -and yellowish stones of the first variety emit a brilliant orange light -when being ground on a copper wheel charged with diamond dust, and -the golden stones of the intermediate type glow with a fine orange -incandescence in the flame of a bunsen burner; the latter phenomenon is -supposed to be due to the presence of thoria. - -The leaf-green stones almost invariably show a series of parallel bands -in the interior. - -Zircons vary from 5s. to 15s. a carat, but exceptional stones may be -worth more. - -By far the finest stones come from Ceylon. The colourless stones are -there known as ‘Matura diamonds,’ and the hyacinth includes garnet -(hessonite) of similar colour, which is found with it in the same -gravels. The stones are always water-worn. Small hyacinths and deep-red -stones come from Expailly, Auvergne, France, and yellowish-red crystals -are found in the Ilmen Mountains, Orenburg, Russia. Remarkably fine -red stones have been discovered at Mudgee, New South Wales, and -yellowish-brown stones accompany diamond at the Kimberley mines, South -Africa. - - - - - CHAPTER XXVII - - CHRYSOBERYL - - (_Chrysolite_, _Cat’s-Eye_, _Cymophane_, _Alexandrite_) - - -Chrysoberyl has at times enjoyed fleeting popularity on account of the -excellent cat’s-eyes cut from the fibrous stones, and in the form of -alexandrite it meets with a steadier, if still limited, demand. It is a -gem-stone that is seldom met with in ordinary jewellery, although its -considerable hardness befits it for all such purposes. - -Chrysoberyl is in composition an aluminate of beryllium corresponding -to the formula BeAl_{2}O_{4}, and is therefore closely akin to spinel. -It usually contains some ferric and chromic oxides in place of alumina, -and ferrous oxide in place of beryllia, and it is to these accessory -constituents that its tints are due. Other gem-stones containing the -uncommon element beryllium are phenakite and beryl. Pale yellowish -green, the commonest colour, is supposed to be caused by ferrous -oxide; such stones are known to jewellers as chrysolite (Plate XXVII, -Fig. 12). Cat’s-eyes (Plate XXIX, Fig. 1) have often also a brownish -shade of green. The bluish green and dark olive-green stones known -as alexandrite (Plate XXVII, Figs. 11, 13) differ in appearance so -markedly from their fairer sisters that their common parentage seems -almost incredible. The dull fires that glow within them, and the -curious change that comes over them at night, add a touch of mystery to -these dark stones. Chromic oxide is held responsible for their colour. -The cat’s-eyes are, of course, always cut _en cabochon_, but otherwise -chrysoberyl is faceted. - -The name of the species is composed of two Greek words, χρυσός, golden, -and βήρυλλος, beryl, and etymologically more correctly defines the -lighter-coloured stones, which were, indeed, at one time the only kind -known. Chrysolite from χρυσὁς, golden, and λίθος, stone, has much the -same significance. This name is preferred by jewellers, but in science -it is applied to an entirely different species, which is known in -jewellery as peridot. Cymophane, from κῦμα, wave, and φαίνειν, appear, -refers to the peculiar opalescence characteristic of cat’s-eyes; it -is sometimes used to designate these stones, but does not find a -place within the vocabulary of jewellery. Alexandrite is named after -Alexander II, Czar of Russia, because it first came to light on his -birthday. That circumstance, coupled with its display of the national -colours, green and red, and its at one time restriction to the mining -district near Ekaterinburg, renders it dear to the heart of all loyal -Russians. - -Chrysoberyl crystallizes in the orthorhombic system, and occurs in -rather dull, complex crystals, which are sometimes so remarkably -twinned, especially in the variety called alexandrite, as to simulate -hexagonal crystals. In keeping with the crystalline symmetry it -is doubly refractive and biaxial, having two directions of single -refraction. The least and the greatest of the principal indices of -refraction may have any values between 1·742 and 1·749, and 1·750 and -1·757, respectively, the maximum amount of double refraction remaining -always the same, namely, 0·009. The mean principal refractive index -is close to the least; the sign of the double refraction is therefore -positive, and the shadow-edge corresponding to the lower index, as -seen in the refractometer, has little, if any, perceptible motion -when the stone is rotated. The converse is the case with corundum; -the sign is negative, and it is the shadow-edge corresponding to the -greater refractive index that remains unaltered in position on rotation -of the stone. This test would suffice to separate chrysoberyl from -yellow corundum, even if the refractive indices of the former were -not sensibly lower than those of the latter. Also, the dichroism of -chrysolite is stronger than that of yellow sapphires. In alexandrite -this phenomenon is most prominent; the absorptive tints, columbine-red, -orange, and emerald-green, corresponding to the three principal -optical directions, are in striking contrast, and the first differs -so much from the intrinsic colour of the stone as to be obvious to -the unaided eye, and is the cause of the red tints visible in a cut -stone. The curious change in colour of alexandrite, from leaf-green to -raspberry-red, that takes place when the stone is seen by artificial -light, is due to a different cause, as has been pointed out above (p. -54). The effect is illustrated by Figs. 11, 13 on Plate XXVII, which -represent a fine Ceylon stone as seen by daylight and artificial light; -the influence of dichroism may be noticed in the former picture. The -specific gravity of chrysoberyl varies from 3·68 to 3·78. In hardness -this species ranks above spinel and comes next to corundum, being -given the symbol 8½ on Mohs’s scale. Certain stones contain a multitude -of microscopic channels arranged in parallel position. When the stones -are cut with their rounded surface parallel to the channels, a broadish -band of light is visible running across the stone at right angles -to them, and suggests the pupil of a cat’s eye, whence the common -name for the stones. The fact that the channels are hollow causes an -opalescence, which is absent from the quartz cat’s-eye. - -The most important locality for the yellowish chrysoberyl is the rich -district of Minas Novas, Minas Geraes, Brazil, where it occurs in the -form of pebbles, and excellent material is also supplied by Ceylon, -in both crystals and rounded pebbles. Other places for chrysolite are -Haddam, Connecticut, and Greenfield, Saratoga County, New York, in -the United States, and recently in the gem-gravels near the Somabula -Forest, Rhodesia. Ceylon supplies some of the best cat’s-eyes. -Alexandrite was first discovered, as already stated, at the emerald -mines near Ekaterinburg, in the Urals; but the supply is now nearly -exhausted. A poorer quality comes from Takowaja, also in the Urals. -Good alexandrite has come to light in Ceylon, and most of the stones -that are placed on the market at the present day have emanated from -that island. The Ceylon stones reach a considerable size, often as much -as from 10 to 20 carats in weight; the Russian stones have a better -colour and are more beautiful, but they are less transparent, and -rarely exceed a carat in weight. Good chrysolite may be worth from 10s. -to £2 a carat, and cat’s-eye runs from £1 to £4 a carat, depending -upon the quality. Alexandrites meet with a steady demand in Russia, and -fine stones are scarce; flawless stones about a carat in weight are -worth as much as £30 a carat, and even quite ordinary stones fetch £4 a -carat. - -From Ceylon, that interesting home of gems, have originated some -magnificent chrysoberyls, including a superb chrysolite, 80¾ carats -in weight, and another, a splendid brownish yellow in colour and -very even in tint, and two large alexandrites, green in daylight and -a rich red by night, weighing 63⅜ and 28-23/32 carats. The finest -cut chrysolite existing is probably the one exhibited in the Mineral -Gallery of the British Museum (Natural History). Absolutely flawless -and weighing 43¾ carats, it was formerly contained in the famous Hope -collection, and is described on page 56 and figured on Plate XXI of -the catalogue prepared by B. Hertz, which was published in 1839; the -weight there given includes the brilliants and the ring in which it -was mounted. It is shown, about actual size, in Plate XXVII, Fig. 12. -A magnificent cat’s-eye, 35·5 by 35 mm. in size, which also formed -part of the Hope collection, was included in the crown jewels taken -from the King of Kandy in 1815. The crystalline markings in the cut -stone are so arranged that the lower half shows an altar overhung by a -torch. The stone has been famous in Ceylon for many ages. It was set -in gold with rubies cut _en cabochon_. Two fine Ceylon alexandrites of -exceptional merit, weighing 42 and 26¾ carats, are also exhibited in -the Mineral Gallery of the British Museum (Natural History). The former -is illustrated in Plate XXVII, Figs. 11, 13, as seen in daylight and in -artificial light. - - - - - CHAPTER XXVIII - - QUARTZ - - (_Rock-Crystal_, _Amethyst_, _Citrine_, _Cairngorm_, _Cat’s-Eye_, - _Tiger’s-Eye_) - - -Although the commonest and, in its natural form, the most easily -recognizable of mineral substances, quartz nevertheless holds a not -inconspicuous position among gem-stones, because, as amethyst (Plate -XXVII, Fig. 7), it provides stones of the finest violet colour; -moreover, the yellow quartz (Plate XXVII, Fig. 5) so ably vies with the -true topaz that it is universally known to jewellers by the name of the -latter species, and is too often confounded with it, and the lustrous, -limpid rock-crystal even aspires to the local title of ‘diamond.’ -For all purposes where a violet or yellow stone is required, quartz -is admirably suited; it is hard and durable, and it has the merit, -or possibly to some minds the drawback, of being moderate in price. -Despite its comparative lack of ‘fire,’ rock-crystal might replace -paste in rings and buckles with considerable advantage from the point -of view of durability. The chatoyant quartz, especially in the form -known as tiger’s-eye, will for beauty bear comparison with the true -cat’s-eye, which is a variety of chrysoberyl. Except that cat’s-eye is -cut _en cabochon_, quartz is step- or sometimes brilliant-cut. - -Ranking with corundum next to diamond as the simplest in composition -of the gem-stones, quartz is the crystallized form of silica, oxide -of silicon, corresponding to the formula SiO_{2}. When pure, it -is entirely devoid of the faintest trace of colour and absolutely -water-clear. Such stones are called rock-crystal, and it is easy to -understand why in early days it was supposed to represent a form -of petrified water. It is these brilliant, transparent stones that -are, when small, known in many localities as ‘diamonds.’ Before -the manufacture of glass was discovered and brought to perfection, -rock-crystal was in considerable use for fashioning into cups, vases, -and so forth. The beautiful tints characterizing quartz are due to the -usual metallic oxides. To manganese is given the credit of the superb -purple or violet colour of amethyst, which varies considerably in -depth. Jewellers are inclined to distinguish the deep-coloured stones -with the prefix ‘oriental,’ but the practice is to be deprecated, since -it might lead to confusion with the true oriental amethyst, which is -a purple sapphire, one of the rarest varieties of corundum. Quartz -of a yellow hue is properly called citrine, but, as already stated, -jewellers habitually prefer the name ‘topaz’ for it, and distinguish -the true topaz by the prefix Brazilian—not a very happy term, since -both the yellow topaz and the yellow quartz occur plentifully in -Brazil. Sometimes the yellow quartz is termed occidental, Spanish, -or false topaz. Stones with a brownish or smoky tinge of yellow are -called cairngorm, or Scotch topaz. The colour of the yellow stones -is doubtless due to a trace of ferric oxide. Stones of a smoky brown -colour are known as smoky-quartz. Rose-quartz, which is rose-red or -pink in colour and hazy in texture, is comparatively rare; strange -to say, it has never been found in distinct crystals. The tint, -which may be due to titanium, is fugitive, and fades on exposure to -strong sunlight. In milky quartz, as the name suggests, the interior -is so hazy as to impart to the stone a milky appearance. It has -frequently happened that quartz has crystallized after the formation -of other minerals, with the result that the latter are found inside -it. Prase, or mother-of-emerald, which at one time was supposed to -be the mother-rock of emerald, is a quartz coloured leek-green by -actinolite fibres in the interior. Specimens containing hair-like -fibres of rutile—the so-called _flêches d’amour_—are common in mineral -collections, and are sometimes to be seen worked. When enclosing a -massive, light-coloured, fibrous mineral, the stones have a chatoyant -effect, and display, when suitably cut, a fine cat’s-eye effect; in -tiger’s-eye the enclosed mineral is crocidolite, an asbestos, the -original blue hue of which has been changed to a fine golden-brown -by oxidation. Quartz which contains scales of mica, hematite, or -other flaky mineral has a vivid spangled appearance, and is known -as aventurine; it has occasionally been employed for brooches or -similar articles of jewellery. Rainbow-quartz, or iris, is a quartz -which contains cracks, the chromatic effect being the result of the -interference of light reflected from them; it has been artificially -produced by heating the stone and suddenly cooling it. - -The name of the species is an old German mining term of unknown -meaning which has been in general use in all languages since the -sixteenth century. Amethyst is derived from ἀμέθυστος, not drunken, -possibly from a foolish notion that the wearer was exempt from the -usual consequences of unrestrained libations. Pliny suggests as an -alternative explanation that its colour approximates to, but does not -quite reach, that of wine. Aventurine, from _aventura_, an accident, -was first applied to glass spangled with copper, the effect being -said to have been accidentally discovered owing to a number of copper -filings falling into a pot of molten glass in a Venetian factory. - -[Illustration: FIG. 79.—Quartz Crystal.] - -Quartz belongs to the hexagonal system of crystalline symmetry, -and crystallizes in the familiar six-sided prisms terminated by -six inclined, often triangular, faces (Fig. 79); twins are common, -though they are not always obvious from the outward development. In -accordance with the symmetry the refraction is double, and there is one -direction of single refraction, namely, that parallel to the edge of -the prism. The ordinary refractive index has the value 1·544, and the -extraordinary 1·553, and since the latter is the greater, the sign of -the double refraction is positive. The double refraction is small in -amount, but is large enough to enable the apparent doubling of certain -of the opposite edges of a faceted stone to be perceptible when viewed -with a lens through the table-facet. The dichroism of the deep-coloured -stones is quite distinct. Quartz has only about the same amount of -colour dispersion as ordinary glass, and lacks, therefore, ‘fire.’ -The application of strong heat tends, as usual, to weaken or drive -off the colour. Thus the dense smoky-quartz found in Spain, Brazil, -and elsewhere is converted into stones of a colour varying from light -yellow to reddish brown according to the amount and duration of the -application. In the case of amethyst the colour is changed to a deep -orange, or entirely driven off if the temperature be high enough. Its -density is very constant, varying only from 2·654 to 2·660; the purest -stones are the lightest. To it has been assigned the symbol 7 on Mohs’s -scale of hardness. - -To physicists quartz is one of the most interesting of minerals because -of its power of rotating, to an extent depending upon the thickness of -the section, the plane of polarization of a beam of light traversing it -in a direction parallel to the prism edge. It appears, moreover, from -a study of the pyro-electric and general physical characters, that its -molecular structure has a helical arrangement, which, like all screws, -may have a right- or left-handed character. Amethyst is, in fact, -invariably composed of separate twin individuals, alternately right- -and left-handed; in some remarkable crystals the section at right -angles to the prism edge is composed of triangular sectors, alternately -of different hands and of different tints—purple and white. To the -twinning is due the rippled fracture and the feathery inclusions so -characteristic of amethyst. - -Besides its use for ornamental purposes, quartz finds a place as the -material for lenses intended for delicate photographic work, because -its transparency to the ultra-violet light is so much greater than -that of glass. Spectacle lenses made of it are in demand, because they -are not liable to scratches, and retain, therefore, their polish -indefinitely. When fused in the oxyhydrogen flame, quartz becomes a -silica glass, of specific gravity 2·2 and hardness 5 on Mohs’s scale, -which has proved of great service for laboratory ware, because it -withstands sudden and unequal heating without any danger of fracture; -it has also in fine threads been invaluable for delicate torsion work, -because it acquires not the smallest amount of permanent twist, in this -respect being superior to the finest silk threads. - -Clear rock-crystal fetches little more than the cost of the cutting; -citrine and amethyst are worth from 1s. to 5s. a carat, depending -upon the quality and size of the stone; smoky-quartz is practically -valueless; rose-quartz realizes less than 1s. a carat; and the value -of cat’s-eye is also small—only 1s. to 2s. 6d. a carat. Tiger’s-eye at -one time commanded as much as 25s. a carat, but the supply exceeded the -demand, with the consequent collapse in the price. - -Beautiful, brilliant, and limpid rock-crystal is found in various parts -of the world: in the Swiss Alps, at Bourg d’Oisans in the Dauphiné -Alps, France, in the famous Carrara marble, in the Marmaros Comitat -of Hungary, and in the United States, Brazil, Madagascar, and Japan. -Small lustrous stones, known in their localities as ‘Isle of Wight,’ -‘Cornish,’ or ‘Bristol diamonds,’ are found in our own country. Brazil -supplies stones out of which have been cut the clear balls used in -crystal-gazing. The finest amethysts come from Brazil—especially the -State of Rio Grande do Sul—and from Uruguay, India, and the gem-gravels -of Ceylon; good stones also occur at Ekaterinburg, in the Ural -Mountains. A splendid Brazilian amethyst, weighing 334 carats, and -two Russian stones—one hexagonal in contour, weighing 88 carats, and -the other, a deep purple in colour with a circular table, weighing 73 -carats—are exhibited in the British Museum (Natural History). Cairngorm -is known from the place of that name in Banffshire, Scotland, whence -fine specimens have emanated; it is a gem much valued in that country. -Fine cairngorm has also originated from Pike’s Peak, Colorado. Splendid -yellow stones have had their birth in the States of Minas Geraes, São -Paulo, and Goyãz, of Brazil—especially in the last. The fine Spanish -smoky-quartz, which, as already stated, turns yellow on heating, comes -from Hinojosa, in the Province of Cordova. The delicate rose-quartz -is known at Bodenmais in Bavaria, Paris in Maine, United States, and -Ekaterinburg in the Ural Mountains. The finest cat’s-eyes are found in -India and Ceylon, and are high in favour with the natives. Greenish -stones of an inferior quality are brought from the Fichtelgebirge in -Bavaria, and are sold as ‘Hungarian cat’s-eyes,’ despite the fact -that no such stone occurs in Hungary—another instance of jewellers’ -disdain for accuracy. Tiger’s-eye occurs in considerable quantity in -the neighbourhood of Griquatown, Griqualand West, South Africa. A -silicified crocidolite, in which the blue colour is retained, comes -also from Salzburg, and is known as sapphire- or azure-quartz, or -siderite. - -Certain of the pebbles found on the seashore of our coasts, especially -off the Isle of Wight and North Wales, cut into attractive, clear -stones, more or less yellow in colour; but examples suitable for the -purpose are not so numerous as might be supposed, and do not reward any -casual search. _Les affaires sont les affaires._ The local lapidary, -instead of explaining that the pebbles brought to him are not worth -cutting, finds it more convenient and profitable to substitute for -them other, inferior and badly cut, stones, bought by the gross, or -even paste stones; the customer, on the other hand, is contented with -a pretty bauble, and is not grateful for the information that it might -have been obtained for a fraction of the sum paid. - - - - - CHAPTER XXIX - - CHALCEDONY, AGATE, ETC. - - -Chalcedony and agate, and their endless varieties, are composed -mainly of silica, but the separate individual crystals are so small -as to be invisible to the unaided eyesight, and occasionally are -so extremely minute that the structure is almost amorphous. The -colour and appearance vary greatly, depending upon the impurities -contained in the stone, and, since both have been made a criterion for -differentiation of types, a host of names have come into use, none of -which are susceptible of strict definition. On the whole, these stones -may be divided into two groups: chalcedony, in which the structure is -concretionary and the colour comparatively uniform, and agate, in which -the arrangement takes the form of bands, varying greatly in tint and -colour. - -The refraction, though double in the individual, is irregular over the -stone as a whole, and the indices approximate to 1·550. The specific -gravity ranges from 2·62 to 2·64, depending upon the impurities -present. The degree of hardness is about the same as that of quartz, -namely, 7 on Mohs’s scale. All kinds are more or less porous, and -stones of a dull colour are therefore artificially tinted after being -worked. - -The term chalcedony, derived from χαλκηδών the name of a town in -Asia Minor, is usually confined to stones of a greyish tinge. Stones -artificially coloured an emerald green have been cut and put upon -the market as ‘emeraldine.’ Carnelian is a clear red chalcedony, -and sard is somewhat similar, but brownish in tint. Chrysoprase is -apple-green in colour, nickel oxide being supposed to be the agent. -Prase (cf. p. 240), which is a dull leek-green in hue, may also in -part be referred here; the name comes from πράσμον, a leek. Plasma, -which may have the same derivation, is a brighter leek-green. Jasper -is a chalcedony coloured blood-red by iron oxide, while bloodstone is -a green chalcedony spotted with jasper; they are popular stones for -signet rings. Flint, an opaque chalcedony, breaks with a sharp cutting -edge, and was much in request with early man as a tool or a weapon; its -property of giving sparks when struck with steel rendered it invaluable -before the invention of matches. Hornstone is somewhat similar, but -more brittle, while chert is a flinty rock. - -Agate, named after the river Achates in Sicily, where it was found at -the time of Theophrastus, has a peculiar banded structure, the bands -being usually irregular in shape, following the configuration of the -cavity in which it was formed. Moss-agate, or mocha-stone, contains -moss-like inclusions of some fibrous mineral. Onyx is an agate with -regular bands, the layers having sharply different colours; when black -and white, it has, in days gone by, been employed for cameos. Sardonyx -is similar in structure, but red and white in colour. Agate is used in -delicate balances for supporting the steel knife-edges of the balance -itself and of the panholders, and is largely employed—especially when -artificially coloured—for umbrella handles and similar articles. - -Chalcedony and agate are found the whole world over, but India, and -particularly Brazil, are noted for their fine carnelians and agates. - - - - - CHAPTER XXX - - OPAL - - (_White Opal_, _Black Opal_, _Fire-Opal_) - - -That opal in early times excited keen admiration is evident from -Pliny’s enthusiastic description of these stones: “For in them you -shall see the burning fire of the carbuncle, the glorious purple of the -amethyst, the green sea of the emerald, all glittering together in an -incredible mixture of light.” During much of last century, owing to the -foolish superstition that ill-luck dogs the footsteps of the wearer, -the species lay under a cloud, which has even now not quite dispersed, -but exercises a prejudicial effect upon the fortunes of the stone. It -has, however, recently attracted considerable attention owing to the -discovery of the splendid black opals in Australia; at one moment black -with the darkness of night, at the next by a chance movement glowing -with vivid crimson flame, such stones may justly be considered the most -remarkable in modern jewellery. At the present day opal is divided by -jewellers roughly into two main groups: ‘white’ (Plate XXVII, Fig. 6) -and ‘black’ (Plate XXVII, Fig. 9), according as the tint is light or -dark, fire-opal (Plate XXVII, Fig. 10) standing in a separate category. - -Opal differs from the rest of the principal gem-stones in being -not a crystalline body, but a solidified jelly, and it depends for -its attractiveness upon the characteristic play of colour, known, -in consequence, as opalescence (cf. p. 39), which arises from a -peculiarity in the structure. Opal is mainly silica, SiO_{2}, in -composition, but contains in addition an amount of water varying in -precious opal from 6 to 10 per cent. As the original jelly cooled, it -became riddled throughout with cracks, which were afterwards generally -filled with opal matter, containing a different amount of water, -and therefore differing slightly in refractivity from the original -substance. The structure not being quite homogeneous, each crack has -the same action upon light as a soap-film, and gives rise to precisely -similar phenomena; the thinner and more uniform the cracks, the greater -the splendour of the chromatic display, the particular tint depending -upon the direction in which the stone is viewed. The cracks in certain -opals were not filled up, and therefore contain air. Such stones appear -opaque and devoid of opalescence until plunged into water; they are -consequently known as hydrophane, from ὕδωρ, water, and φαίνεσθαι, to -make appear. Owing to the effect of total-reflection, light was stopped -on the hither side of the cracks before they were filled with water, -which is not far inferior to opal in refractivity; it is surprising how -much water these stones will absorb. - -Opal is colourless when pure, but is nearly always more or less milky -and opaque, or tinted various dull shades by ferric oxide, magnesia -or, alumina. The so-called black opal is generally a dark grey or -blue, and very rarely quite black. That the coloration is not due -to ordinary absorption, but to the action of cracks in the stone, -is shown by the fact that the transmitted light is complementary to -the reflected light; the blue opal is, for instance, a yellow when -held up so that light has passed through it. In many black opals the -opalescent material occurs in far too tiny pieces to be cut separately, -and the whole iron-stained matrix is cut and polished and sold under -the name ‘opal-matrix.’ The reddish and orange-coloured stones known -as fire-opal have pronounced colour and only slight milkiness; they -display the customary opalescence in certain directions. These stones -are often faceted, but otherwise opals are cut _en cabochon_, either -flat or steep—generally the former in brooches and pendants, and the -latter in rings. Opal is somewhat soft, varying from 5 to 6½ on Mohs’s -scale, and is therefore easily scratched. The specific gravity ranges -from 2·10 to 2·20, and the refractive index from 1·444 to 1·464, the -refraction, of course, being always single. It is unwise to immerse -opals in liquids on account of their porosity. - -The name opal comes to us through the Latin _opallus_, which was used -for the same species as understood by the term at the present day, but -the word has a far older origin, which has not been traced. The Romans -also called the mineral _pæderos_, the Greek form of Cupid, a name -applied to all rosy stones. The name cacholong, for the bluish-white -porcelain variety, which is very porous and adheres to the tongue, is -of Tartar origin; the stone is highly valued in the East. - -The oldest mines, which up to quite a recent date were the only -extensive deposit of opal known, were at Cserwenitsa, near Kashau, -in Hungary. From them in all probability emanated the opals known to -the Romans. The opals from this locality were generally quite small, -and large pieces were rare and commanded high prices. The Hungary -mines, however, proved quite unable to compete with the rich fields -at White Cliffs, New South Wales, in spite of the efforts that were -made to depreciate and exclude from the market the new stones, and at -the present time few of the opals on the market come from them. As so -often happens, the White Cliffs deposit was discovered by accident. -In 1889 a hunter, when tracking a wounded kangaroo, chanced to pick -up an attractively coloured opal. The district is so waterless and -forbidding that, but for such a chance, the opals might have long lain -hidden. They occur in seams in deposits of Cretaceous Age in a variety -of ways, filling cavities in rocks or sandstones, or cracks in wood, or -replacing wood, saurian bones, and some spiky mineral, which may have -been glauberite. In recent years, another rich deposit was discovered -farther north, on both sides of the boundary between Queensland and -New South Wales. The field is remarkable for the darkness of its -opals, which are called ‘black opal’ in contradistinction to the -lighter-coloured stones previously known. From Lightning Ridge in -New South Wales come stones stained deep black which quite merit the -designation black opal. The sandstone in which they are found is -rich in iron, and this is no doubt responsible for the deepness of -their tint. Mexico is noted for the fire-opal, which is found at -Esperanza, Queretaro, and Zimapan; but other kinds of opal also are -found at these places. - -[Illustration: _PLATE XXVIII_ - -OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES] - -The price of opal varies greatly, according to the intrinsic colour -and the uniformity and brilliance of the opalescence. Common opal can -be bought at as low a rate as 1s. a carat, while black opal ranges -from 10s. to £8 a carat; but a good dark stone displaying a flaming -opalescence commands a fancy figure, fine stones of this class being -exceedingly rare. Fire-opal enjoys only a limited popularity now, -though a few years ago it was in some demand; the price runs from 2s. -to 10s. a carat. - - - - - CHAPTER XXXI - - FELSPAR - - (_Moonstone, Sunstone, Labradorite, Amazon-Stone_) - - -Though second to none among minerals in scientific interest, whether -regarded from the point of view of their crystalline characters or -the important part they play in the formation of rocks, the group -included under the general name felspar occupies but a humble place -in jewellery. It consists of three distinct species, orthoclase, -albite, and anorthite, which are silicates of aluminium, and potassium, -sodium, or calcium, corresponding to the formulae KAlSi_{3}O_{8}, -NaAlSi_{3}O_{8}, and CaAl_{2}Si_{2}O_{8} respectively, and also of -species intermediate in composition between albite and orthoclase, or -albite and anorthite. While differing in crystalline symmetry, all -are characterized by two directions of cleavage which are nearly at -right angles to one another. The double refraction, which is slight in -amount, is biaxial in character and variable in sign. The values of the -least and greatest of the indices of refraction range between 1·52 and -1·53, and 1·53 and 1·55 respectively, the double refraction at the same -time varying from 0·007 to 0·012. The specific gravity lies between -2·48 and 2·66, and the hardness ranges between the degrees 6 and 7 on -Mohs’s scale. - -Moonstone (Plate XXIX, Fig. 4), which is mainly pure orthoclase, alone -is at all common in jewellery. It forms such an admirable contrasting -frame for large coloured stones that it deserves greater popularity; -no doubt the cheapness of the stones militates against their proper -appreciation. The milky, bluish opalescence from which they take their -name is caused by the reflection of light at the thin twin-lamellæ -of which the structure is composed. They are always cut more or less -steeply _en cabochon_. The finest stones were at one time cut from -the felspar that came from the St. Gothard district in Switzerland -and was in consequence known as adularia from the neighbouring Adular -Mountains, somewhat incorrectly, since none occurs at the latter -locality. At the present day practically all the moonstones on the -market come from Ceylon. They run in price from £3 to £20 per oz. (28 -grams). - -Sunstone is a felspar containing flakes of hematite or goethite which -impart a spangled bronze appearance to the stones. Good material occurs -in parts of Norway. The remarkable sheen of labradorite or blue felspar -has its origin in the interference of light at lamellar surfaces in -the interior; the uniformity of the colour over comparatively large -areas testifies to the regularity of the lamellar arrangement. The -finest specimens were brought from the Isle of St. Paul off the coast -of Labrador, where they were first discovered in 1770; large masses -also occur on the coast itself. Amazon-stone is an opaque green felspar -which occurs in the Ilmen Mountains, Orenburg, Russia, and at Pike’s -Peak, Colorado, United States. It obtains its name from the Amazon -River, where, however, none has ever been found; there may have been -some confusion with a jade or similar stone. - -Occasionally clear colourless felspar has been faceted, and then -closely resembles rock-crystal. A careful determination of the -refractive indices and the specific gravity serves to discriminate -between them. - -[Illustration: _PLATE XXIX_ - - 1. CAT’S EYE - 2. PERIDOT - 3. PERIDOT - 4. MOONSTONE - 5. HESSONITE - 6. PYROPE - 7. DEMANTOID - 8. ALMANDINE - 9. SPODUMENE - 10. KUNZITE - 11. HIDDENITE - 12. ZIRCON - 13. ZIRCON - 14. ZIRCON - 15. ANDALUSITE - 16. NEPHRITE - 17. TURQUOISE - 18. JADEITE - -GEM-STONES] - - - - - CHAPTER XXXII - - TURQUOISE, ODONTOLITE, VARISCITE - - -Of all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a -prominent place in jewellery and can aspire to rank with the precious -stones. The colour varies from a sky-blue or a greenish blue to a -yellowish green or apple-green. Only the former tints, which are at -the same time the rarer, are in general demand, and they possess the -great advantage of harmonizing with the tint of the gold setting. -The blue colours are, especially in the case of the Siberian stones, -by no means permanent, and fade in course of time. Turquoise is -amorphous and seldom crystalline, and is therefore somewhat porous; it -should consequently never be immersed in liquids or be contaminated -with greasy and dirty matter lest the dreaded change of colour be -brought about. The stones are translucent in thin sections, and a -good observation is possible with the refractometer if the back of -the stone is flat and polished, since only the section immediately -adjacent to the instrument is concerned; the refractive index is about -1·61. The specific gravity varies from 2·75 to 2·89. Turquoise has a -hardness of slightly under 6 on Mohs’s scale, and takes a good polish, -which is fairly durable, since on account of the comparative opacity -of the stones scratches on the surface are not very noticeable. -In composition it is a complex phosphate of aluminium and copper, -corresponding to the formula CuOH.[6Al(OH)_{2}].H{5}.(PO_{4})_{4}, -with ferric oxide replacing some alumina. The blue colour is due to -the copper constituent, and the predominance of iron may cause the -greenish shades; but the water contained in the stones plays no mean -part, since they turn a dirty green when it is driven off. The faded -colour can sometimes be restored by immersion of the stone in ammonia -and subsequent application of grease, but the effect is not lasting. -Attempts are sometimes made to improve inferior stones by impregnating -them with Berlin blue, but with only qualified success. Turquoises are -said to be affected by the perspiration from the skin. - -The name of the species comes from a French word meaning Turkish, and -arises from the fact that the gem-stone first reached Europe by way of -Turkey. Another, but less obvious, suggestion is that it is derived -from the Persian name for the species, _piruzeh_. Our turquoise and -other phosphates of similar appearance were probably known to Pliny -under the three names _callais_, _callaina_, and _callaica_. - -The finest turquoise still comes from the famous mines near Nishapur in -the Persian province of Khorassan, where it was known in very ancient -times; it is found with limonite filling the cracks and cavities -in a brecciated porphyritic trachyte. Pieces of the turquoise and -limonite from here are sometimes cut without removal of the latter, -and sold as ‘turquoise-matrix,’ when the precious stones are too tiny -to be worth separate working. It also occurs at Serbâl in the Sinai -Peninsula. Among the more recent localities may be mentioned Los -Cerillos Mountains, New Mexico; Sierra Nevada, Nevada, where pale blue -and green stones are found; San Bernardino County, California, where -again the stones are rather pale; and Arizona, where it occurs in pale -greenish-blue stones. - -Some of the stones that have been seen are not the true turquoise but -odontolite, or bone turquoise, which consists of the teeth and bones -of mastodon or other extinct animals, phosphate of iron being the -colouring material. These stones may easily be recognized by their -organic structure, which is clearly visible if viewed with a strong -lens or under the microscope. Moreover, odontolite invariably contains -some calcium carbonate, and effervescence takes place if it be touched -with hydrochloric acid. Turquoise dissolves in hydrochloric acid, but -without effervescence, and since it contains copper, a fine blue colour -is imparted to the solution by the addition of ammonia. Odontolite has -a higher specific gravity, 3·0 to 3·5, but lower hardness, 5 on Mohs’s -scale. - -Variscite, the hydrated phosphate of aluminium, corresponding to the -formula AlPO_{4} + 2H_{2}O, is found in masses resembling a greenish -turquoise, but it is much softer, being only 4 on Mohs’s scale. The -specific gravity is 2·55. Round nodular masses of variscite are found -in Utah. - - - - - CHAPTER XXXIII - - JADE - - -Though not usually accounted precious among European nations or in -Western civilization in general, jade was held in extraordinary -esteem by primitive man, and was fashioned by him into ornaments and -utensils, often of considerable beauty, and even at the present day -it ranks among the Chinese and Japanese peoples above all precious -stones; indeed, the Chinese word _Yu_ and the Japanese words _Giyuku_ -or _Tama_ signify both jade and precious stones in general. According -to the Chinese, jade is the prototype of all gems, and unites in -itself the five cardinal virtues—_Jin_, charity; _Gi_, modesty; _Yu_, -courage; _Ketsu_, justice; and _Chi_, wisdom. When powdered and mixed -with water, it is supposed to be a powerful remedy for all kinds of -internal disorders, to strengthen the frame and prevent fatigue, to -prolong life, and, if taken in sufficient quantity just before death, -to prevent decomposition. - -Jade is a general term that includes properly two distinct mineral -species, nephrite or greenstone, and jadeite, which are very similar -in appearance, both being fibrous and tough in texture, and more or -less greenish in colour; but it is also applied to other species such -as saussurite, californite, bowenite, and plasma, which have somewhat -similar characters. The word jade is a corruption of the Spanish -_pietra di hijada_, kidney-stone, in allusion to its supposed efficacy -in diseases of that organ. - -Nephrite or greenstone (Plate XXIX, Fig. 16) is the commoner of -the two jades. It is closely allied to the mineral hornblende, a -silicate of magnesium, iron, and calcium corresponding to the formula -Ca(Mg,Fe)_{3}(SiO_{3})_{4}, the magnesia being replaceable by ferrous -oxide. Microscopic examination shows that the structure consists of -innumerable independent fibres foliated or matted together, the former -character giving rise to a slaty and the latter to a horny appearance -in the stone as seen by the unaided eye. The colour varies from grey -to leaf- and dark-green, the tint deepening as the relative amount of -iron in the composition increases, and brown tints result from the -oxidation of the iron along cracks in the stone. The hardness is 6½ on -Mohs’s scale; nephrite is therefore about as hard as ordinary glass and -softer than quartz. When polished, it always acquires a greasy lustre. -The specific gravity ranges from 2·9 to 3·1. The least and greatest of -the principal refractive indices are 1·606 and 1·632 respectively, the -double refraction being biaxial and negative; the coloured fibres also -display dichroism. All these differential effects are, however, masked -in the stone because of the irregularity of the aggregation. Nephrite -is fusible before the blowpipe, but only with difficulty. Its name is -derived from the Greek word νεφρός, kidney, the allusion being the same -as for jade. - -Many of the prehistoric implements found in Mexico and in the Swiss -Lake Habitations are composed of nephrite, but it is uncertain where -the mineral was obtained. Much of the material used by the Chinese -at the present time comes from spots near the southern boundary of -Eastern Turkestan, especially in the valleys of the rivers Karakash and -Yarkand in the Kwen Lun range of mountains; it is also found farther -north at the river Kashgar. It occurs in various provinces of China, -namely, Shensi, Kwei Chau, Kwang Tung, Yunnan, and Manchuria. Gigantic -waterworn boulders have been found in the Government of Irkutsk, near -Lake Baikal, in eastern Siberia, the first discovery being made in the -bed of the Onot stream by the explorer and prospector J. P. Alibert, -in 1850. A large boulder of this kind, weighing over half a ton (1156 -lb., or 524·5 kg.), is exhibited in the Mineral Gallery of the British -Museum (Natural History). An enormous mass, weighing over 2 tons (4718 -lb., or 2140 kg.), was discovered at Jordansmühl, Silesia, by Dr. -G. F. Kunz, and is now in the magnificent collection of jade formed -by Mr. Heber R. Bishop. Beautiful greenstone occurs in New Zealand, -particularly in the Middle Island. The Maoris have long used it for -various useful and ornamental purposes, the most common being indicated -by their general name for the species, _punamu_, axe-stone; _kawakawa_ -is the ordinary green variety, a fine section of which is shown on the -wall of the Mineral Gallery of the British Museum (Natural History), -while _inanga_, a grey variety, and _kahurangi_, a pale-green and -translucent variety, are rare and highly prized. - -Jadeite (Plate XXIX, Fig. 18) is by far the rarer of the two jades, -and is the choicest gem with the Chinese. In composition it is a -silicate of sodium and aluminium with the formula NaAl(SiO_{3})_{2}, -corresponding to the lithium mineral spodumene (p. 265). It has the -same toughness and greasy lustre as nephrite, but is harder, being -represented by the symbol 7 on Mohs’s scale, and thus only slightly, -if at all, softer than quartz. The other characters are also higher; -the specific gravity is about 3·34, and the least and greatest of the -principal refractive indices are 1·66 and 1·68, the double refraction -being biaxial and negative. The colour varies from white to almost -an emerald green, the latter being especially prized, and often the -green colour runs in streaks through the white. Jadeite fuses readily -before the blowpipe to blebby glass, more easily than is the case with -nephrite. - -The finest jadeite comes from the Mogaung district in Upper Burma, -where it is found in boulders and also with albite in dykes in a -dark-green serpentine. The export trade to China, which absorbs -practically the whole of the output, is exceedingly valuable, and -realizes nearly as much as the produce of the ruby mines. Jadeite is -also found in the Shensi and Yunnan provinces of China, and in Tibet. - - • • • • • - -A few words may be said about the other jade-like minerals. Saussurite, -which is named after H. B. de Saussure, has resulted from the -decomposition of a felspar, and is nearly akin to the mineral zoisite. -It has the customary toughness of structure, and is greenish grey to -white in colour. Its specific gravity is about 3·2, and hardness 6½ to -7 on Mohs’s scale. It occurs near Lake Geneva. Bowenite is a green -serpentine (p. 289) which is found at Smithfield, Rhode Island, U.S.A., -and in New Zealand and Afghanistan. Californite and plasma are compact -varieties of idocrase (p. 275) and chalcedony (p. 247) respectively. -Verdite is a stone of rich green colour which is found in the form of -large boulders in the North Kaap River, South Africa; it is composed of -green mica (fuchsite) and some clayey matter. - -Jade has of recent years been imitated in glass, but the latter is -recognizable by its vitreous lustre and inferior hardness, and sooner -or later by its frangibility. - - - - - CHAPTER XXXIV - - SPODUMENE, IOLITE, BENITOITE - - - SPODUMENE - - (_Kunzite_, _Hiddenite_) - -Till a few years ago scarcely known outside the ranks of mineralogists, -spodumene suddenly leaped into notice in 1903 upon the discovery of the -lovely lilac-coloured stones (Plate XXIX, Fig. 10) at Pala, San Diego -County, California; they shortly afterwards received the name kunzite -after the well-known expert in gems, Dr. G. F. Kunz. The stones were -found here in a pegmatite dyke, and were of all shades, ranging from -pale pink to deep lilac, and at times as much as 150 carats in weight. -Paler kunzite occurs with beryl and tourmaline at Coahuila Mountain -in Riverside County, California, and colourless stones have recently -come to light in Madagascar. Kunzite is remarkable for its wonderful -dichroism; the beautiful violet tint that springs out in one direction -comes with greater surprise because of the uninteresting yellowish -tints in other directions. Unlike spodumene in general, kunzite is -phosphorescent under the influence of radium. - -The emerald-green variety (Plate XXIX, Fig. 11), named hiddenite after -Mr. W. E. Hidden, who discovered in 1881 the only known occurrence, in -Alexander County, North Carolina, would no doubt have become popular -had the supply of material not been so very limited; few stones were -found, and the variety has never come to light elsewhere. The colour is -supposed to be due to chromic acid. Hiddenite being also dichroic, the -tint varies with the direction. - -Spodumene is ordinarily rather a pale yellowish in hue, and, as its -name (which is derived from σποδίος, ash-coloured) suggests, is not -very attractive. Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are -found in Brazil and Madagascar. - -The species is interesting scientifically because it contains the -rare element lithium; it is a silicate of aluminium and lithium, -corresponding to the formula LiAl(SiO_{3})_{2}. The double refraction -is biaxial in character and positive in sign, the least and greatest -of the refractive indices being 1·660 and 1·675; the specific gravity -is 3·185, and hardness 6½ to 7 on Mohs’s scale. Spodumene has an easy -cleavage, and the cut stones call therefore for careful handling, lest -they be flawed or fractured. Two faceted stones, a beautiful kunzite -and a fine hiddenite, weighing 60 and 2½ carats respectively, are -exhibited in the British Museum (Natural History). - - - IOLITE - -Known also by various other names—cordierite, dichroite, and -water-sapphire (_saphire d’eau_)—this species owes its interest -to the remarkable dichroism characterizing it, the principal -colours—smoky-blue and yellowish white—being in such contrast as to -be obvious to the unaided eye. The stones that are usually worked have -intrinsically a smoky-blue colour, and are found in waterworn masses -in the river-gravels of Ceylon, whence is the origin of the name -water-sapphire. Iolite, from ἴον, violet, and λίθος, stone, refers to -the colour; cordierite is named after Cordier, a French geologist, who -first studied the crystallography of the species; and dichroite, of -course, alludes to the most prominent character of the species. - -Iolite is a silicate of aluminium and of magnesium and iron -corresponding to the formula H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37}. -The double refraction is small in amount, biaxial in character, and -negative in sign, the least and greatest of the refractive indices -being 1·543 and 1·551; the specific gravity is 2·63, and hardness 7 on -Mohs’s scale. Iolite, if used, is worked and polished; it is seldom -faceted. A large worked piece, weighing 177 grams, which was formerly -in the Hawkins Collection, is exhibited in the British Museum (Natural -History). - - - BENITOITE - -The babe among gem-stones, benitoite first saw the light of day a few -years ago, early in 1907. It occurs with the rare mineral neptunite, -which was previously known only from Greenland, in narrow veins of -natrolite in Diablo Range near the head-waters of the San Benito River, -San Benito County, California. Despite careful search the species has -not been found except within the original restricted area. To science -it is interesting both because of its composition, a silico-titanate -of barium, corresponding to the formula BaTiSi_{3}O_{9}, and because -its crystals belong to a class of crystalline symmetry which has -hitherto not been represented among minerals. The double refraction -is uniaxial, and since the ordinary index of refraction is 1·757 and -the extraordinary 1·804, it is positive in sign and large in amount, -namely, 0·047. The stones are characterized by strong dichroism, the -colour corresponding to the ordinary ray being white, and to the -extraordinary greenish blue to indigo depending upon the tint of the -stone. To obtain the best effect the stone must therefore be cut with -the table-facet parallel to the crystallographic axis. The specific -gravity is 3·65, and hardness 6½ on Mohs’s scale. When first discovered -the species was supposed to be sapphire, and many stones were cut and -sold as such. It is, however, much softer than sapphire, and is readily -distinguished by its optical characters, since it possesses greater -double refraction and of differing sign, so that, when tested with the -refractometer, the shadow-edge corresponding to the lower index of -refraction remains fixed in the case of benitoite, whereas the contrary -happens with sapphire. Benitoite also, unlike sapphire, fuses easily -to a transparent glass. Its blue colour, which is supposed to be due -to a small amount of free titanic acid present, appears to be stable. -Several stones as large as 1½ to 2 carats in weight have been found. -The largest of all, perfectly flawless, weighs just over 7 carats, and -is remarkable because it is about three times the next largest in point -of weight; it is the property of Mr. G. Eacret, of San Francisco. - - - - - CHAPTER XXXV - - EUCLASE, PHENAKITE, BERYLLONITE - - - EUCLASE - -This species comes near beryl in chemical composition, being a -silicate of aluminium and beryllium corresponding to the formula -Be(AlOH)SiO_{4}, and closely resembles aquamarine in colour and -appearance when cut. Owing to the rarity of the mineral good specimens -command high prices for museum collections, and it is seldom worth -while cutting it for jewellery. It derives its name from its easy -cleavage, εὖ easily, and κλάσις fracture. The double refraction is -biaxial in character and positive in sign, the least and greatest of -the refractive indices being 1·651 and 1·670 respectively; the specific -gravity is 3·07, and the hardness 7½ on Mohs’s scale. The colour is -usually a sea-green, but sometimes blue. Euclase occurs with topaz at -the rich mineral district of Minas Novas, Minas Geraes, Brazil, and has -also been found in the Ural district, Russia. - - - PHENAKITE - -Another beryllium mineral, phenakite owes its name to the frequency -with which it has been mistaken for quartz, being derived from φέναξ, -deceiver. The clear, colourless crystals, somewhat complex in form, -have at times been cut, but they lack ‘fire,’ and despite their -brilliant lustre meet with little demand. The composition is a silicate -of beryllium corresponding to the formula Be_{2}SiO_{4}. The double -refraction is uniaxial, and since the ordinary, 1·652, is less than -the extraordinary index, 1·667, it is positive in sign; the specific -gravity is 2·99, and the hardness is almost equal to that of topaz, -being about 7½ to 8 on Mohs’s scale. - -Fine stones have long been known near Ekaterinburg in the Ural -Mountains, and have recently been discovered in Brazil. - - - BERYLLONITE - -As its name suggests, this mineral also contains beryllium, being -a soda phosphate corresponding to the formula NaBePO_{4}. Clear, -colourless stones, which occur at Stoneham, Maine, U.S.A., have been -cut, but the lack of ‘fire,’ the easy cleavage, and comparative -softness, the symbol being 5½ on Mohs’s scale, unfit it for use in -jewellery. The double refraction is biaxial in character and negative -in sign, the least and the greatest of the refractive indices being -1·553 and 1·565 respectively. - - - - - CHAPTER XXXVI - - ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE, - SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE - - - ENSTATITE - - (‘_Green Garnet_’) - -The small green stones which accompany the diamond in South Africa have -been cut and put on the market as ‘green garnet.’ They are, however, -in no way connected with garnet, but belong to a mineral species -called enstatite, which is a silicate of magnesium corresponding to -the formula MgSiO_{3}; the green colour is due to a small amount of -ferrous oxide which replaces magnesia. The double refraction is biaxial -in character and positive in sign, the least and greatest of the -refractive indices being 1·665 and 1·674 respectively; the specific -gravity ranges from 3·10 to 3·13, and the hardness is only about 5½ -on Mohs’s scale. The dichroism is perceptible, the twin-colours being -yellowish and green, and, as usual, is more pronounced the deeper the -colour of the stone. There is also a good cleavage in two different -directions. - -With increasing percentage amount of iron enstatite passes into -hypersthene. The colour becomes a dark brownish green, and an increase -takes place in the physical constants, the least and greatest of the -refractive indices attaining to 1·692 and 1·705 respectively, and -the specific gravity ranging from 3·4 to 3·5. Hypersthene is never -sufficiently transparent for faceting, but when spangled with small -scales of brookite it is sometimes cut _en cabochon_. - -The name enstatite is derived from ἐνστάτης, an opponent, referring to -the infusibility of the mineral before the blowpipe, and hypersthene -comes from ὑπερσθένος, very tough. - -An altered enstatite, leek-green in colour and with nearly the -composition of serpentine (p. 289), has been cut _en cabochon_. It has -much lower specific gravity, only 2·6, and lower hardness, 3½ to 4 on -Mohs’s scale. It is named bastite from Baste in the Harz Mountains, -where it was first discovered. - - - DIOPSIDE - -This species, which is also known as malacolite and alalite, provides -stones of a leaf-green colour which have occasionally been cut. It -is a silicate of calcium and magnesium corresponding to the formula -MgCa(SiO_{3})_{2}, but usually contains in place of magnesia some -ferrous oxide, to which it owes its colour; with increase in the -percentage amount of iron the colour deepens and the physical constants -change. The double refraction is large in amount, 0·028, biaxial -in character, and positive in sign. The least and greatest of the -refractive indices corresponding to the stones suitable for jewellery -range about 1·671 and 1·699 respectively, but they may be as high as -1·732 and 1·750 in the two cases. The specific gravity varies from 3·20 -to 3·38, and the hardness from 5 to 6 on Mohs’s scale. Dichroism is -noticeable in deep-coloured stones, but is not very marked. - -The name diopside comes from δίς, double, and ὄψις, appearance, in -allusion to the effect resulting from the double refraction; malacolite -is derived from μαλακός, soft, because the mineral is softer than the -felspar associated with it; and alalite is named after the principal -locality, Ala Valley, Piedmont, Italy. - - - KYANITE - -Kyanite, also known as disthene, is interesting for two reasons. Its -structure is so grained in character that the hardness varies in the -same stone from 5 to 7 on Mohs’s scale; it can therefore be scratched -by a knife in some directions, but not in others (p. 79). It has the -same chemical composition as andalusite, both being silicates of -aluminium corresponding to the formula Al_{2}SiO_{5}, but possesses -very different physical characters, a fact which shows how large -a share the molecular grouping has in determining the aspect of -crystallized substances. It is biaxial with small negative double -refraction, the least and greatest of the refractive indices being -1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in -sky-blue prismatic crystals, whitish at the edges, in schist near St. -Gothard, Switzerland. It is seldom cut. - -Kyanite is derived from its colour, κύανος blue, and disthene, from its -variable hardness, δίς, twice, and σθένος, strong. - - - ANDALUSITE - -Andalusite bears no resemblance whatever to kyanite, although, as has -been stated above, the composition of the two species is essentially -the same. It is usually light bottle-green in colour, and more rarely -brown and reddish. Its extreme dichroism is its most remarkable -character, the twin colours being olive-green and red. The reddish -gleams that are reflected from the interior are in sharp contrast with -the general colour of the stone, and impart to it a weird effect (Plate -XXIX, Fig. 15). Cut stones are often confused with tourmalines, and -can, indeed, only be distinguished from the latter with certainty by -noting on the refractometer the smaller amount of double refraction -and the difference in its character. The least and greatest of the -refractive indices are 1·62 and 1·643 respectively, and the double -refraction, 0·011, about half that of tourmaline, is biaxial and -negative; the specific gravity is 3·18, and hardness 7½ on Mohs’s scale. - -Good stones are found at Minas Novas, Minas Geraes, Brazil, and in -the gem-gravels of Ceylon. It was first known from the province of -Andalusia, Spain, whence is the origin of its name. - - - IDOCRASE - - (_Vesuvianite_, _Californite_) - -Idocrase, also known as vesuvianite, is occasionally found in the -form of transparent, leaf-green, and yellowish-brown stones which, -when cut, may be mistaken for diopside and epidote respectively, but -are distinguishable from both by the extreme smallness of their -double refraction. Californite is a compact variety which has all the -appearances of a jade; its colour is green, or nearly colourless with -green streaks. - -In composition idocrase is a silicate of aluminium and calcium, the -precise formula of which is uncertain, but may be— - - (Ca,Mn,Mg,Fe)_{2}[(Al,Fe)(OH,F)]Si_{2}O_{7}. - -The double refraction, which is uniaxial in character and negative in -sign, may be less than 0·001, and never exceeds 0·006, so that it is -not easily detected with the refractometer, even in sodium light. The -refractive indices vary enormously in value, from 1·702 to 1·726 for -the ordinary, and from 1·706 to 1·732 for the extraordinary ray. The -specific gravity varies from 3·35 to 3·45, and the hardness is about 6½ -on Mohs’s scale. - -The name idocrase, from εἴδος, form, and κρᾶσις, mixture, was assigned -to the species by Haüy, but his reasons have little meaning at the -present day. The other names are taken from the localities where the -species and the variety were first discovered. - -Bright, green crystals come from Russia, and also from Ala Valley, -Piedmont, and Mount Vesuvius, Italy. Californite is found in large -masses in Siskiyon and Fresno Counties, California. - - - EPIDOTE - - (_Pistacite_) - -Epidote often possesses a peculiar shade of yellowish green, similar -to that of the pistachio-nut—hence the origin of its alternative -name—which is unique among minerals, though scarcely pleasing enough -to recommend it to general taste. Its ready cleavage renders it liable -to flaws; nevertheless, it is occasionally faceted. The name epidote, -from ἐπίδοσις, increase, was given to it by Haüy, but not on very -precise crystallographical grounds. - -In composition this species is a silicate of calcium and aluminium, -with some ferric oxide in place of alumina, corresponding to the -complex formula, Ca_{2}(Al,Fe)_{2}[(Al,Fe)OH](SiO_{4})_{3}. It -occurs in monoclinic, prismatic crystals richly endowed with natural -faces. The colour deepens with increase in the percentage amount of -iron, and the stones become almost opaque. The double refraction is -large in amount, 0·031, biaxial in character, and negative in sign. -The dichroism is conspicuous in transparent stones, the twin-tints -corresponding to the principal optical directions being green, brown, -and yellow. The values of the least and greatest of the refractive -indices given by transparent stones are 1·735 and 1·766 respectively; -the specific gravity varies from 3·25 to 3·50, and the hardness from 6 -to 7 on Mohs’s scale. - -Transparent crystals have come from Knappenwand, Untersulzbachtal, -Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs, -Norway. Magnificent, but very dark, crystals were discovered about ten -years ago on Prince of Wales Island, Alaska. - - - SPHENE - - (_Titanite_) - -The clear, green, yellow, or brownish stones provided by this species -would be welcomed in jewellery because of their brilliant and -almost adamantine lustre, but, unfortunately, they are too soft to -withstand much wear, the hardness being only 5½ on Mohs’s scale. In -composition sphene is a silico-titanate of calcium corresponding to -the formula CaTiSiO_{5}, and in this respect comes near the recently -discovered gem-stone, benitoite. The refractive indices lie outside -the range of the refractometer, the values of the least and the -greatest of the refractive indices varying from 1·888 and 1·917 to -1·914 and 2·053 respectively. It is to this high refraction that it -owes its brilliant lustre. The double refraction, which is biaxial in -character and positive in sign, is so large that the apparent doubling -of the opposite edges of a cut stone when viewed through one of the -faces is obvious to the unaided eye (cf. p. 41). Cut stones have -additional interest on account of the vivid dichroism displayed, the -twin-tints, colourless, yellow, and reddish yellow, corresponding to -the three principal optical directions, being in strong contrast. The -specific gravity ranges from 3·35 to 3·45. The negative test with the -refractometer (cf p. 26), the softness, and the large amount of double -refraction suffice to distinguish this species from gem-stones of -similar appearance. - -The name sphene, from σφήν, wedge, alludes to the shape of the natural -crystals. The alternative name is obviously due to the fact that the -species contains titanium. - -Good stones have come from the St. Gothard district, Switzerland. - - - AXINITE - -Called axinite from the shape of its crystals—ἀξίνη, axe—this species -supplies small, clear, clove-brown, honey-yellow, and violet stones -which can be cut for those who care for a stone out of the ordinary. -The composition is a boro-silicate of aluminium and calcium, with -varying amounts of iron and manganese, corresponding to the formula -(Ca,Fe)_{3}Al_{2}(B.OH)Si_{4}O_{15}. Axinite is interesting on -account of its strong dichroism, the twin-tints corresponding to the -principal optical directions being violet, brown, and green. The double -refraction is biaxial in character and negative in sign, the least and -greatest of the refractive indices being 1·674 and 1·684; the specific -gravity is 3·28, and hardness about 6½ to 7, or rather under that of -quartz. - -The best examples have been found at St. Cristophe, Bourg d’Oisans, in -the Dauphiné, France. Violet axinite is a novelty that has come within -recent years from Rosebery, Montagu County, Tasmania. - - - PREHNITE - -This species, which is named after its discoverer, Colonel Prehn, is -found in nodular, yellow and oil-green stones, of which the latter -have very occasionally been cut. It is a little soft, the hardness -being only 6 on Mohs’s scale. The double refraction is large in amount, -0·033, biaxial in character, and positive in sign, the least and the -greatest of the refractive indices being 1·616 and 1·649 respectively; -the specific gravity varies from 2·81 to 2·95. In composition prehnite -is a silicate of aluminium and calcium corresponding to the formula -H_{2}Ca_{2}Al_{2}(SiO_{4})_{3}. - -The best material has been found at St. Cristophe, Bourg d’Oisans, -Dauphiné, France. - - - APATITE - -This interesting mineral is found occasionally in attractive green, -blue, or violet stones, but is unfortunately too soft for extensive use -in jewellery, the hardness being only 5 on Mohs’s scale. In composition -it is a fluo-chloro-phosphate of calcium, corresponding to the formula -Ca_{4}[Ca(F,Cl)](PO_{4})_{3}. When pure, it is devoid of colour, -the tints being due to the presence of small amounts of tinctorial -agents. The double refraction is uniaxial in character and negative in -sign, the ordinary index being 1·642 and the extraordinary 1·646; the -specific gravity varies from 3·17 to 3·23. The dichroism is usually -feeble, but sometimes is strong; for instance, in the stones from the -Burma ruby mines (yellow, blue-green). A cut stone might be mistaken -for tourmaline, but is distinguished by its softness, or, when tested -on the refractometer, by its inferior double refraction. It received -its name from ἀπατάειν, deceive, because it was wrongly assigned to at -least half a dozen different species in early days. Moroxite is a name -sometimes given to blue-green apatite. - -Beautiful violet stones are found at Ehrenfriedersdorf, Saxony; -Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin County, -Maine, U.S.A.; and blue stones come from Ceylon. - - - DIOPTASE - -Though of a pretty, emerald-green colour, dioptase has never been -found in large enough crystals for gem purposes, and it is, moreover, -rather soft, the hardness being only 5 on Mohs’s scale, and has an -easy cleavage. In composition it is a hydrous silicate of copper -corresponding to the formula CuH_{2}SiO_{4}. The double refraction, -which is large in amount, is uniaxial in character, and positive in -sign, the ordinary refractive index being 1·667 and the extraordinary -1·723. Its colour and softness distinguish it from peridot or diopside, -which have about the same refractivity. The name was assigned to the -species by Haüy, from διὰ, through, and ὄπτομαι, see, because the -cleavage directions were distinguishable by looking through the stone. - -Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at -Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli, -near Comba, in the French Congo. - - - - - CHAPTER XXXVII - - CASSITERITE, ANATASE, PYRITES, HEMATITE - - - CASSITERITE - -Though usually opaque, this oxide of tin, corresponding to the formula -SnO_{2}, has occasionally, but very rarely, been found in small, -transparent, yellow and reddish stones suitable for cutting. The lustre -is adamantine. The refraction is uniaxial in character and positive -in sign, the ordinary index being 1·997 and extraordinary 2·093. The -specific gravity is high, ranging from 6·8 to 7·1. The hardness is on -the whole less than that of quartz, being about 6 to 7 on Mohs’s scale. - - - ANATASE - -This mineral, which is one of the three crystallized forms of titanium -oxide, TiO_{2}, occurs often in small, brown, transparent stones which -occasionally find their way into the market. The lustre is adamantine. -The refraction is uniaxial in character and negative in sign, the -extraordinary index being 2·493 and ordinary 2·554. The specific -gravity varies from 3·82 to 3·95, and the hardness is about 5½ to 6 on -Mohs’s scale. - - - PYRITES, HEMATITE - -These two metallic minerals were employed in ancient jewellery. The -former, sulphide of iron, FeS_{2}, is brass-yellow in colour, and has -a specific gravity 5·2, and hardness 6½ on Mohs’s scale. It is found, -when fresh, in brilliant cubes. The latter, oxide of iron, Fe_{2}O_{3}, -has a black metallic lustre, but, when powdered, is red in colour—a -mode of distinguishing it from other minerals of similar appearance. -Its specific gravity is 5·3, and hardness 6½ on Mohs’s scale. In modern -times it has been cut in spherical form to imitate black pearls, but -can easily be recognized by its greater density and hardness. Hematite -is used for signet stones, often with an intaglio engraving. - - - - - CHAPTER XXXVIII - - OBSIDIAN, MOLDAVITE - - -Two forms of natural glass have been employed for ornamental purposes. -Obsidian results from the solidification without crystallization of -lava, and corresponds in composition to a granite. The structure -is seldom clear and transparent, and usually contains inclusions -or streaks. The colour is in the mass jet-black, but smoky in thin -fragments, and occasionally greenish. Its property of breaking with a -keen cutting edge, in the same way as ordinary glass, rendered it of -extreme utility to primitive man, who was ignorant of the artificial -substance. The refraction is, of course, single, and the refractive -index approximates to 1·50. The specific gravity varies from 2·3 to -2·5. The hardness is 5 on Mohs’s scale, the same as ordinary glass. - -Obsidian is obtained wherever there has been volcanic activity. Vast -mines of great antiquity exist in the State of Hidalgo, Mexico. - -Moldavite, which differs in no respect from ordinary green -bottle-glass, is of interest on account of its problematical origin. -Its occurrence in various parts of Bohemia and Moravia cannot be -explained as the result of volcanic agency. It may possibly be the -product of old and forgotten glass factories which at one time existed -on the site. Even meteorites have been suggested as the source. The -physical characters are the same as those of ordinary glass: refraction -single, index 1·51; specific gravity 2·50 and hardness 5½ on Mohs’s -scale. Moldavite also passes under the names of bottle-stone, or -water-chrysolite. A natural glass of the same character has been found -in water-worn fragments in Ceylon, and has been sold as peridot, which -it resembles in colour, but is readily distinguished from it by its -very different physical properties. - - - - - PART II—SECTION C - - ORNAMENTAL STONES - - - - - CHAPTER XXXIX - - FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE, AZURITE, - MALACHITE, THULITE, MARBLE, APOPHYLLITE, CHRYSOCOLLA, - STEATITE OR SOAPSTONE, MEERSCHAUM, SERPENTINE - - -Space will not permit of more than a few words concerning the more -prominent of the numerous mineral species which are employed for -ornamental purposes in articles of virtu or in architecture, but which -for various reasons cannot take rank as gem-stones. - -Fluor, a beautiful mineral which is found in its greatest perfection in -England, has enjoyed well-deserved popularity when worked into vases -or other articles. The finest material, deep purple in colour, known -as ‘Blue John,’ came from Derbyshire, but the supply is now exhausted. -The crystallized examples, from Durham, Devonshire, and Cornwall, form -some of the most attractive of museum specimens. The crystals take the -shape of cubes, often twinned, and have an easy octahedral cleavage. -The refraction is single, the index being 1·433. Fluor is noted for its -property of appearing of differing colour by reflected and transmitted -light, and the phenomenon is in consequence known as fluorescence. The -specific gravity is 3·18, and the hardness 4 on Mohs’s scale. Owing to -its low refraction and softness, fluor is not suitable for jewellery. -Clear colourless material is in demand for particular lenses of -microscope objectives. - -The lovely blue stone known as lapis lazuli has since the earliest -times been applied to all kinds of decorative purposes, for mosaic -and inlaid work and as the material for vases, boxes, and so on, and -was the original sapphire of the ancients. When ground to powder it -furnishes a fine blue paint, but it has now been entirely superseded -for this purpose by an artificial product. Although to the eye so -homogeneous and uniform in structure, lapis lazuli has been shown -by microscopic examination to be composed of calcite coloured by -three blue minerals in varying proportions. All three belong to the -cubic class of symmetry, and are mainly soda aluminium silicates -in composition; their hardness varies from 5 to 6 on Mohs’s scale. -Lazurite, Na_{4}(NaS_{3}.Al)Al_{2}Si_{3}O_{12}, has specific gravity -varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite, -(Na_{2},Ca)_{2}(NaSO_{4},Al)Al_{2}Si_{3}O_{12}, is about the same in -specific gravity, 2·4 to 2·5, but slightly harder, 5½ to 6; while -sodalite, Na_{4}(AlCl)Al_{2}Si_{3}O_{12}, is the lightest in density, -2·14 to 2·30, with hardness 5½ to 6, and has a refractive index 1·483. - -By far the oldest mines are in the Badakshan district of Afghanistan, -a few miles above Firgamu in the valley of the Kokcha, a branch of -the Oxus, where ruby and spinel are found. It is also found at the -southern end of Lake Baikal, Siberia, and in the Chilian Andes. - -Sodalite occurs in beautiful blue masses at Dungannon, Hastings County, -Ontario, Canada, and at Litchfield, Maine, U.S.A. They make excellent -polished stones. - -Violane, a massive, dark violet-blue diopside from San Marcel, -Piedmont, Italy, also makes a handsome polished stone. - -Rhodonite, silicate of manganese, MnSiO_{3}, possesses a fine red -colour, and makes an attractive stone when cut and polished. It has -very slight biaxial double refraction, the refractivity being about -1·73; the specific gravity is 3·6, and hardness 6. It is found in large -masses near Ekaterinburg in the Ural Mountains, and is quarried as an -ornamental stone. - -Both the copper carbonates, azurite or chessylite, and malachite, make -effective polished stones. The latter is also worked into various -ornamental objects; it occurs in fibrous masses, the grained character -of which look well in the polished section. Its colour is a bright -green, to which it owes its name, from μαλακή, mallows. Its composition -is represented by the formula CuCO_{3}.Cu(OH)_{2}, and it is the more -stable form, since azurite is frequently found altered to it. It -has biaxial double refraction, and the indices are about 1·88; the -specific gravity is 4·01, and hardness about 3½ to 4 on Mohs’s scale. -It is found in large masses at the copper mines of Nizhni Tagilsk in -the Ural Mountains, where it is mined as an ornamental stone; it also -accompanies the copper ores in many parts of the world, for instance -Cuba, Chili, and Australia. Azurite, so called on account of its -beautiful blue colour, is rarer, but, unlike malachite, is generally in -the form of crystals. Beautiful specimens have come from Chessy, near -Lyons, France, and Bisbee, Arizona, U.S.A. The composition corresponds -to the formula 2CuCO_{3}, Cu(OH)_{2}. The specific gravity is 3·80, and -hardness about 3½ to 4. - -Chrysocolla occurs in blue and bluish-green earthy masses, with -an enamel-like texture, which in some instances can be worked and -polished. Being the result of the decomposition of copper ores, it -varies considerably in hardness, ranging from 2 to 4 on Mohs’s scale. -Its composition approaches to the formula CuSiO_{3}.2H_{2}O, but it -invariably contains impurities. It is very light, the density being -only about 2·2. - -Steatite, or soapstone, is a massive foliated silicate of magnesium -corresponding to the formula H_{2}Mg_{3}Si_{4}O_{12}, which is one of -the softest of mineral substances, representing the degree 1 on Mohs’s -scale, but in massive pieces is harder owing to the intermixture of -other substances with it. It has a peculiar greasy feeling to the -touch, due to its softness. The specific gravity is about 2·75. The -Chinese carve images out of the yellowish and brownish pieces. - -Meerschaum, a silicate of magnesium corresponding to the formula -H_{4}Mg_{2}Si_{3}O_{10}, is familiar to every smoker as a material for -pipe-bowls. It is very light, the specific gravity being only 2·0, and -soft, the hardness being about 2 to 2½ on Mohs’s scale. When found, -it is pure white in colour, and answers to its name, a German word -signifying sea-foam. It comes from Asia Minor. - -Serpentine has been largely used for decorative purposes, as well -as for cameos and intaglios, and formed most of the famous ‘verde -antique.’ Being the result of the decomposition of other silicates -it varies enormously in appearance and characters, but the most -attractive stones are a rich oil-green in colour and resemble jade. The -composition approximates to the formula H_{4}Mg_{3}Si_{2}O_{9}, but it -invariably contains other elements. The hardness varies from 2½ to 4 -on Mohs’s scale, according to the minerals contained in the stone; the -specific gravity is about 2·60 and the refractivity 1·570. - -The beautiful rose-red stone, thulite, makes a handsome decorative -stone. It has nearly the same composition as epidote (p. 275), and like -it has strong dichroism, the principal colours being yellow, light -rose, and deep rose. The colour is due to manganese. Its refractive -index is about 1·70, specific gravity 3·12, and hardness 6 to 6½ on -Mohs’s scale; it possesses an easy cleavage. Fine specimens come from -Telemark, Norway, and it is therefore called after the old name for -Norway, Thule. - -Marble is a massive calcite, carbonate of lime, with the formula -CaCO_{3}. When pure it is white, but it is usually streaked with other -substances which impart a pleasing variety to its appearance. It is -always readily recognized by the immediate effervescence set up when -touched with a drop of acid. Calcite is highly doubly refractive (cf. -p. 40), the extraordinary index being 1·486, and ordinary 1·658, a -difference of 0·172; the specific gravity is 2·71, and hardness 3 on -Mohs’s scale. Lumachelle, or fire-marble, is a limestone containing -shells from which a brilliant, fire-like chatoyancy is emitted -when light is reflected at the proper angle. It sometimes resembles -opal-matrix, but is easily distinguished by its lower hardness and by -its effervescent action with acid. Choice specimens come from Bleiberg -in Carinthia, and from Astrakhan. - -Apophyllite has not many characters to commend it, being at the best -faintly pinkish in colour, and always imperfectly transparent. It is -a hydrous silicate of potassium and calcium with the complex formula -(H,K)_{2}Ca(SiO_{3})_{2}.H_{2}O. Its refractivity is about 1·535, -specific gravity 2·5, and hardness 4½ on Mohs’s scale; it possesses -an easy cleavage. It occurs in the form of tetragonal crystals at -Andreasberg in the Harz Mountains, and in the Syhadree Mountains, -Bombay, India. - - - - - PART II—SECTION D - - ORGANIC PRODUCTS - - - - - CHAPTER XL - - PEARL, CORAL, AMBER - - -Although none of the substances considered in this chapter come within -the strict definition of a stone, since they are directly the result of -living agency, yet pearl at least cannot be denied the title of a gem. -Both pearl and coral contain calcium carbonate in one or other of its -crystallized forms, and both are gathered from the sea; but otherwise -they have nothing in common. Amber is of vegetable origin, and is a -very different substance. - - - PEARL - -From that unrecorded day when some scantily clothed savage seeking for -succulent food opened an oyster and found to his astonishment within -its shell a delicate silvery pellet that shimmered in the light of a -tropical sun, down to the present day, without intermission, pearl has -held a place all its own in the rank of jewels. Though it be lacking in -durability, its beauty cannot be disputed, and large examples, perfect -in form and lustre, are sufficiently rare to tax the deepest purse. - -The substance composing the pearl is identical with the iridescent -lining—mother-o’-pearl or nacre, as it is termed—of the shell. -Tortured by the intrusion of some living thing, a boring parasite, -a worm, or a small fish, or of a grain of sand or other inorganic -substance, and without means to free itself, the mollusc perforce -neutralizes the irritant matter by converting it into an object of -beauty that eventually finds its way into some jewellery cabinet. Built -up in a haphazard manner and not confined by the inexorable laws of -intermolecular action, a pearl may assume any and every variety of -shape from the regular to the fantastic. It may be truly spherical, -egg- or pear-shaped—pear-drops or pear-eyes, as they are termed—or it -may be quite irregular—the so-called baroque or barrok pearls. The -first is the most prized, but a well-shaped drop-pearl is in great -demand for pendants or ear-rings. The colour is ordinarily white, or -faintly tinged yellowish or bluish, and somewhat rarely, salmon-pink, -reddish, or blackish grey. Perfect black pearls are valuable, but not -as costly as the finest of the white. Though not transparent, pearl is -to a varying extent translucent, and its characteristic lustre—‘orient’ -in the language of jewellery—is due to the same kind of interaction -of light reflected from different layers that has been remarked upon -in the case of opal and certain other stones. The translucency varies -in degree, and some jewellers speak of the ‘water’ of pearls just as -in the case of diamonds. If a pearl be sliced across the middle and -the section be examined under the microscope, it will be seen that -the structure consists of concentric shells and resembles that of an -onion. These shells are alternately composed of calcium carbonate in -its crystallized form, aragonite, and of a horny organic matter known -as conchiolin, and they evidently represent the result of intermittent -growth. Because of their composite character, pearls have a specific -gravity ranging from 2·65 to 2·69-2·84-2·89 in the case of pink -pearls—which is appreciably less than that of aragonite, 2·94: the -hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the -arrangement of the mineral layers is approximately parallel is evinced -by the distinctness of the shadow-edges shown on examination with the -refractometer. Pearls require very careful handling, both because they -are comparatively soft and therefore apt to be scratched, and because -they are chemically affected by acids, and even by the perspiration -from the skin. Acids attack only the calcium carbonate, not the -organic matter; the well-known story therefore of Cleopatra dissolving -a valuable pearl in vinegar, which is moreover, too weak an acid to -effect the solution quickly, must not be accepted too literally. -Pearls are not cut like stones, and therefore as soon as the precious -bloom has once gone, nothing can be done to revive it. Attempts are -sometimes made in the case of valuable pearls to remove the dull skin -and lay bare another iridescent layer underneath, but the operation is -exceedingly delicate. Even with the best of care pearls must in process -of time perish owing to the decay of the organic constituent. Pearls -that have been discovered in ancient tombs crumbled to dust at a touch, -and those formerly in ancient rings have vanished or only remain as -a brown powder, while the garnets or other stones set with them are -little the worse for the centuries that have passed by. - -The largest known pearl was at one time in the famous collection -belonging to the banker, Henry Philip Hope. Cylindrical in form, with -a slight swelling at one end, it measures 50 mm. (2 inches) in length, -and 115 mm. (4½ inches) in circumference about the thicker, and 83 -mm. (3¼ inches) about the thinner end, and weighs 454 carats. About -three-quarters of it is white in colour with a fine ‘orient,’ and the -remainder is bronze in tint. It is valued at upwards of £12,000. A -large pearl, 300 carats in weight, is in the imperial crown of the -Emperor of Austria, and another, pear-shaped, is in the possession of -the Shah of Persia. A beautiful white India pearl, a perfect sphere in -shape, and 28 carats in weight, is in the Museum of Zosima in Moscow; -it is known as ‘La Pellegrina.’ The ‘Great Southern Cross,’ which -consists of nine large pearls naturally joined together in the shape of -a cross, was discovered in an oyster fished up in 1886 off the beds of -Western Australia. The collection of jewels in the famous Green Vaults -at Dresden contains a number of pearls of curious shapes. - -Large pearls are sold separately, while the small pearls known as -‘seed’ pearls come into the market bored and strung on silk in -‘bunches.’ The unit of weight is the pearl grain, which is a quarter -of a carat, and the rate of price depends on the square of the weight -in grains. The rate per unit or base varies from 6d. to 50s. according -to the shape and quality of the pearl. Spherical pearls command the -best prices, next the pearl-drops, and lastly the buttons; but whatever -the shape, it is imperative that the pearl have ‘orient,’ without which -it is valueless. The cheaper grades of pearls are sold by the carat. - -[Illustration: _PLATE XXX_ - -NATIVES DRILLING PEARLS] - -For use in necklaces and pendants pearls are bored with a steel drill, -and threaded with silk, an easy operation on account of their softness. -They harmonize well with diamonds. Small pearls are often set as a -frame to large coloured stones, to which they form an admirable foil. -Pearls set in rings or anywhere where the upper half alone would show -are generally sawn in halves; ‘button’ pearls find an extensive use in -modern rings. - -Any mollusc, whether of the bi-valve or the uni-valve type, which -possesses a nacreous shell, has the power of producing pearls, -but only two, the pearl-oyster, _Meleagrina margaritifera_, and -the pearl-mussel, _Unio margarifer_, repay the cost of systematic -fishing. The outside of the shell is formed of the horny matter called -conchiolin; while the inside is composed of two coats, of which -the outer consists of alternate layers of conchiolin and calcium -carbonate in its crystallized form, calcite, and the inner of the same -organic matter, but with calcium carbonate in its other crystallized -form, aragonite. The latter coat forms the nacreous lining known as -mother-o’-pearl, which is identical in consistency with pearl, but -somewhat more transparent. The iridescence of mother-o’-pearl is due -not only to the fact that it is composed of a succession of thin -translucent layers, but also to the fact that these layers overlap -like slates on a house, and form a series of fine parallel lines on -the surface; diffraction therefore as well as interference of light -takes place, and a similar diffraction phenomenon is displayed even -by a cast of the inside of the shell. The animal has the property of -secreting calcium carbonate, which it absorbs from the sea-water, in -both its crystallized conditions as well as conchiolin. At the outer -rim it secretes conchiolin, further in calcite, and at the very inside -aragonite. The shape and appearance of a pearl therefore depend on -the position in which the intruding substance is situated within the -shell. The most perfect pearl has been in intermittent motion in the -interior of the mollusc, and has received successive coats according -to the position in which it happened to be. A parasite that bores -into the shell is walled up at the point of entrance, and a wart- or -blister-pearl results. The thinner the successive coats the finer -the lustre. Pearls have even been discovered embedded in the animal -itself. The number of pearls found in a shell depends on the number of -times the living host was compelled to seal up some irritant object, -and may vary from one up to the eighty-seven which are said to have -been found in an Indian oyster. That an oyster thus distinguished has -not led a happy existence is testified by the distorted shape of its -shell, a clue that guides the pearl-fishers in their search. Moreover, -pearl-oysters never have thick nacreous shells, and on the other hand -molluscs with fine mother-o’-pearl seldom contain pearls. - -Beautiful white and silvery pearls are found in a small oyster that -lives at a depth of 6 to 13 fathoms (11-24 m.) in the Gulf of Manaar, -off the coast of Ceylon. About seven-eighths, however, of the pearls -that come into the market are obtained from a larger oyster which -has its home on the Arabian coast of the Persian Gulf. These famous -fisheries have been known since very early times. The pearls found -here are more yellowish than those from Ceylon, but are nevertheless -of excellent quality. The pearl fisheries off the north-west coast of -Western Australia and off Venezuela are also not unimportant, and fine -black pearls have been supplied by molluscs from the Gulf of Mexico. - -[Illustration: _PLATE XXXI_ - -METAL FIGURES OF BUDDHA INSERTED IN A PEARL-OYSTER] - -[Illustration: _PLATE XXXII_ - -FIG. 1 - -FIG. 2 - -SECTIONS OF CULTURE PEARL - -FIG. 1. IN THE OYSTER. FIG. 2. WHEN FINISHED. - -A. PEARLY DEPOSIT. B. PIECE OF MOTHER-O’-PEARL INSERTED IN THE OYSTER. -C. OUTER SHELL OF THE OYSTER. D. MOTHER-O’-PEARL BACK ADDED.] - -The Chinese have long made a practice of introducing into the shell of -a pearl-oyster little tin images of Buddha in order that they may be -coated with the nacreous secretion. The Japanese have during recent -years made quite an industry of stimulating the efforts of the mollusc -by cementing small pieces of mother-o’-pearl to the interior surface -of the shell (Plate XXXII, Fig. 1); these ‘culture’ pearls, as they -are termed, are recognizable by examination of the back. About a year -has to elapse before a coating of a tenth of a millimetre is formed, -and another two years must pass before the thickness is doubled. After -removal the piece of mother-o’-pearl, which is now coated with several -nacreous layers, is cemented to a piece of ordinary mother-o’-pearl, -and the lower portion is ground to the usual symmetrical shape (Plate -XXXII, Fig. 2). Blister pearls are often similarly treated. In both -cases, however, the ‘orient’ is deficient in quality. - -The finest mother-o’-pearl is supplied by a mollusc found in the sea -near the islands lying between Borneo and the Philippines, and fine -material is found at Shark Bay and off Thursday Island. - - - CORAL - -Coral ranks far below pearl and meets with but limited appreciation. -It is common enough in warm seas, but the only kind which finds its -way into jewellery is the rose or red-coloured coral—the noble coral, -_Corallium nobile_ or _rubrum_. It consists of the axial skeleton of -the coral polyp, and is built up of hollow tubes fitting one within -the other. The composition is mainly calcium carbonate with a little -magnesium carbonate and a small amount of organic matter. The former -of the mineral substances is in the form of calcite, and the crystals -are arranged in fibrous form radiating at right angles to the axis of -the coral. The specific gravity varies from 2·6 to 2·7, being slightly -under that of calcite, and the hardness is somewhat greater, being -about 3¾ on Mohs’s scale. - -The best red coral is found in the Mediterranean Sea off Algiers and -Tunis in Africa, and Sicily and the Calabrian Coast of Italy. The -industry of shaping and fashioning the coral is carried on almost -entirely in Italy. Coral is usually cut into beads, either round or -egg-shaped, and used for necklaces, rosaries, and bracelets. The best -quality fetches from 20s. to 30s. per carat. - - - AMBER - -This fossil resin, yellow and brownish-yellow in tint, finds an -extensive use as the material for mouthpieces of pipes, cigar and -cigarette-holders, umbrella-handles, and so on, and is even locally -cut for jewellery, although its extreme softness, its hardness being -only 2½ on Mohs’s scale, quite unfits it for such a purpose. It is -only slightly denser than water, the specific gravity being about -1·10. Since the structure is amorphous the refraction is single, the -index being about 1·540. Amber, being a very bad conductor of heat, is -perceptibly warm to the touch. Its property of becoming electrified by -friction attracted early attention, and from the Greek name for it, -ἤλεκτρον, is derived our word electricity. - -Amber is washed up by the sea off the coasts of Sicily and Prussia, -and of Norfolk and Suffolk in England. The finest examples, which are -picked up off the shore of Catania in Sicily, are distinguished by a -fine bluish fluorescence, resembling that seen in lubricating oil; such -pieces command good prices. - -A recent resin, pale yellow in colour, known as kauri-gum, is found in -New Zealand, where it is highly valued. - - - - - TABLES - - - TABLE I - - _Chemical Composition of Gem-Stones_ - - (_a_) ELEMENTS— - - Diamond C - - (_b_) OXIDES— - - Corundum Al_{2}O_{3} - Quartz SiO_{2} - Chalcedony SiO_{2} - Opal SiO_{2}.nH_{2}O - - (_c_) ALUMINATES— - - Spinel MgAl_{2}O_{4} - Chrysoberyl BeAl_{2}O_{4} - - (_d_) SILICATES— - - Phenakite Be_{2}SiO_{4} - Dioptase H_{2}CuSiO_{4} - Peridot Mg_{2}SiO_{4} - Zircon ZrSiO_{4} - Enstatite MgSiO_{3} - Diopside CaMg(SiO_{3})_{2} - Nephrite CaMg_{3}(SiO_{3})_{4} - Sphene CaTiSiO_{5} - Benitoite BaTiSi_{3}O_{9} - Andalusite Al(AlO)SiO_{4} - Kyanite (AlO)_{2}SiO_{3} - Topaz [Al(F,OH)]_{2}SiO_{4} - Epidote Ca_{2}(Al,Fe)_{2}(AlOH)(SiO_{4})_{3} - Euclase Be(AlOH)SiO_{4} - Prehnite H_{2}Ca_{2}Al_{2}(SiO_{4})_{3} - Iolite H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37} - { Hessonite Ca_{3}Al_{2}(SiO_{4})_{3} - _Garnet_ { Pyrope Mg_{3}Al_{2}(SiO_{4})_{3} - { Almandine Fe_{3}Al_{2}(SiO_{4})_{3} - { Andradite Ca_{3}Fe_{2}(SiO_{4})_{3} - Beryl Be_{3}Al_{2}(SiO_{3})_{6} - Spodumene LiAl(SiO_{3})_{2} - Jadeite NaAl(SiO_{3})_{2} - Moonstone KAlSi_{3}O_{8} - Tourmaline{12SiO_{2}.3B_{2}O_{3}.(9-x)[(Al,Fe)_{2}O_{3}].3x[ - {(Fe,Mn,Ca,Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O - Axinite HCa_{3}Al_{2}B(SiO_{4})_{4} - Idocrase (Ca,Mn,Mg,Fe)_{2}(Al,Fe)(OH,F)]Si_{2}O_{7} - - (_e_) PHOSPHATES— - - Beryllonite NaBePO_{4} - Apatite Ca_{5}(F,Cl)(PO_{4})_{3} - Turquoise CuOH.6[Al(OH)_{2}].H_{5}.(PO_{4})_{4} - - - TABLE II - - _Colour of Gem-Stones_ - - _Colourless and White._—Diamond, corundum (white sapphire), topaz, - quartz (rock-crystal), zircon (when ‘fired’), moonstone; rarely - beryl, tourmaline; among the less common species, phenakite, - spodumene (colourless kunzite), beryllonite. - - _Yellow._—Diamond, topaz, corundum (yellow sapphire), quartz - (citrine, Scotch or occidental topaz), tourmaline, zircon, - sphene, spodumene, beryl. - - _Pink and Lilac._—Corundum (pink sapphire), spinel (balas-ruby), - tourmaline (rubellite), topaz (usually when ‘fired’), spodumene - (kunzite), beryl (morganite), quartz (rose-quartz). - - _Red._—Corundum (ruby), garnet (pyrope, almandine), spinel - (balas-ruby), tourmaline (rubellite), zircon, opal (fire-opal). - - _Green._—Beryl (emerald, aquamarine), peridot, corundum, - tourmaline, chrysoberyl (including alexandrite), zircon, garnet - (demantoid); among less common species, spodumene (hiddenite), - euclase, diopside, idocrase, epidote, apatite, obsidian; rarely - diamond; also semi-opaque, turquoise, jade. - - _Blue._—Corundum (sapphire), spinel, topaz, tourmaline, zircon; - among the less common species, kyanite, iolite, benitoite, - apatite; rarely diamond; also semi-opaque, turquoise, lapis - lazuli, sodalite. - - _Violet and Purple._—Quartz (amethyst), corundum (oriental - amethyst), spinel (almandine-spinel), garnet (almandine), - spodumene (kunzite), apatite. - - _Brown._—Diamond, tourmaline, quartz (smoky-quartz); among the less - common species, andalusite, axinite, sphene. - - - TABLE III - - _Refractive Indices of Gem-Stones_[8] - - Opal 1·454 - Moonstone 1·53 1·54 - Iolite 1·543 1·551 - Quartz 1·544 1·553 - Beryllonite 1·553 1·565 - Beryl 1·578 1·585 - Turquoise 1·61 1·65 - Topaz 1·618 1·627 - Andalusite 1·632 1·643 - Tourmaline 1·626 1·651 - Apatite 1·642 1·646 - Phenakite 1·652 1·667 - Euclase 1·651 1·670 - Spodumene 1·660 1·675 - Enstatite 1·665 1·674 - Peridot 1·659 1·697 - Axinite 1·674 1·684 - Diopside 1·685 1·705 - Idocrase 1·714 1·719 - Spinel 1·726 - Kyanite 1·72 1·73 - Epidote 1·735 1·766 - Garnet (Hessonite) 1·745 - Chrysoberyl 1·746 1·753 - Garnet (Pyrope) 1·755 - Benitoite 1·757 1·804 - Corundum 1·761 1·770 - Garnet (Almandine) 1·790 - Zircon (a) 1·815 - Garnet (Demantoid) 1·885 - Sphene 1·901 1·985 - Zircon (b) 1·927 1·980 - Diamond 2·417 - - - TABLE IV - - _Colour-Dispersion of Gem-Stones_[9] - - Moonstone ·012 - Quartz ·013 - Beryl ·014 - Topaz ·014 - Chrysoberyl ·015 - Tourmaline ·017 - Spodumene ·017 - Corundum ·018 - Peridot ·020 - Spinel ·020 - Garnet (Almandine) ·024 - Garnet (Pyrope) ·027 - Garnet (Hessonite) ·028 - Zircon ·038 - Diamond ·044 - Sphene ·051 - Garnet (Demantoid) ·057 - - - TABLE V - - _Character of the Refraction of Gem-Stones_ - - - (_a_) SINGLE— - - Diamond, spinel, garnet, opal. - Diamond and garnet frequently display local double refraction. - - (_b_) UNIAXIAL, POSITIVE— - - Quartz ·009 - Phenakite ·015 - Benitoite ·047 - Zircon (b) ·053 - Quartz exhibits circular polarization. - - (_c_) UNIAXIAL, NEGATIVE— - - Apatite ·004 - Idocrase ·005 - Beryl ·007 - Corundum ·009 - Tourmaline ·025 - - (_d_) BIAXIAL, POSITIVE— - - Chrysoberyl ·007 - Topaz ·009 - Enstatite ·009 - Spodumene ·015 - Euclase ·019 - Diopside ·020 - Peridot ·038 - Sphene ·084 - - (_e_) BIAXIAL, NEGATIVE— - - Moonstone ·006 - Iolite ·008 - Axinite ·010 - Andalusite ·011 - Beryllonite ·012 - Kyanite ·016 - Epidote ·031 - - - TABLE VI - - _Dichroism of Gem-Stones_ - - (_a_) STRONG - - Corundum, tourmaline, alexandrite, spodumene, andalusite, iolite, - epidote, axinite. - - (_b_) DISTINCT - - Emerald, topaz, quartz, peridot, chrysoberyl, enstatite, euclase, - idocrase, kyanite, sphene, apatite. - - (_c_) WEAK - - Beryl, diopside. - - - TABLE VII - - _Specific Gravities of Gem-Stones_ - - Opal 2·15 - Moonstone 2·57 - Iolite 2·63 - Quartz 2·66 - Beryl 2·74 - Turquoise 2·82 - Beryllonite 2·84 - Phenakite 2·99 - Euclase 3·07 - Tourmaline 3·10 - Enstatite 3·10 - Andalusite 3·18 - Spodumene 3·18 - Apatite 3·20 - Axinite 3·28 - Diopside 3·29 - Epidote 3·37 - Peridot 3·40 - Idocrase 3·40 - Sphene 3·40 - Diamond 3·52 - Topaz 3·53 - Spinel 3·60 - Kyanite 3·61 - Garnet (Hessonite) 3·61 - Benitoite 3·64 - Chrysoberyl 3·73 - Garnet (Pyrope) 3·78 - Garnet (Demantoid) 3·84 - Corundum 4·03 - Garnet (Almandine) 4·05 - Zircon (a) 4·20 - Zircon (b) 4·69 - - - TABLE VIII - - _Degrees of Hardness of Gem-Stones_ - - 5. Kyanite (5-7), apatite, lapis lazuli - 5½. Enstatite, beryllonite, sphene - 6. Opal, moonstone, turquoise, diopside - 6½. Spodumene, peridot, garnet (demantoid), benitoite, idocrase, - epidote, axinite, jade (nephrite) - 7. Iolite, quartz, tourmaline, jade (jadeite) - 7¼. Garnet (hessonite, pyrope) - 7½. Beryl, garnet (almandine), zircon, phenakite, euclase, andalusite - 8. Topaz, spinel - 8½. Chrysoberyl - 9. Corundum - 10. Diamond - - - TABLE IX.—DATA - - _Densities of Water and Toluol at Ordinary Temperatures_ - - +-----------------------------+----------+----------+ - | TEMPERATURE | WATER | TOLUOL | - +-----------------------------+----------+----------+ - | Centigrade | Fahrenheit | | | - | | | | | - | 14° | 57·2° | 0·9994 | 0·8697 | - | 15° | 59·0° | 0·9992 | 0·8687 | - | 16° | 60·8° | 0·9990 | 0·8677 | - | 17° | 62·6° | 0·9988 | 0·8667 | - | 18° | 64·4° | 0·9986 | 0·8657 | - | 19° | 66·2° | 0·9985 | 0·8647 | - | 20° | 68·0° | 0·9983 | 0·8637 | - | 21° | 69·0° | 0·9981 | 0·8627 | - | 22° | 71·6° | 0·9979 | 0·8617 | - | 23° | 73·4° | 0·9977 | 0·8607 | - +-----------------------------+----------+----------+ - - 1 English carat = 0·2053 gram - 1 Metric carat = 0·2000 (one-fifth) gram - 1 oz. Av. = 28·35 grams - 1 lb. Av. = 0·4536 kilogram - 1 inch = 25·4 millimetres - 1 foot = 0·3048 metre - 1 yard = 0·9144 metre - 1 mile = 1·6093 kilometre - - - - - INDEX - - - Absorption, 53, 59 - - Absorption spectra, 59 - - Achroite, 220, 221 - - Adularia, 255 - - Agate, 247 - - Akbar Shah diamond, 163 - - Alalite, 272 - - Albite, 254 - - Alexandrite, 54, 60, 233 - Scientific, 122 - - Almandine, 60, 214 - Oriental, 112, 172 - spinel, 112, 204 - - Amazon-stone, 255 - - Amber, 83, 298 - - Amethyst, 239, 242 - Oriental, 111, 172, 239 - - Anatase, 281 - - Andalusite, 274 - - Andradite, 216 - - Anomalous refraction, 47 - - Anorthite, 254 - - Apatite, 279 - - Apophyllite, 290 - - Aquamarine, 184, 193 - - Arizona-ruby, 213 - - Artificial stones, 124 - - Asteria, 38, 177 - - Asterism, 38 - - Australia stones, 154, 174, 182, 195, 213, 216, 227, 232, 252, - 288 - - Austrian Yellow diamond, 165 - - Aventurine, 240, 241 - - Axes, Crystallographic, 9 - Optic, 49 - - Axinite, 278 - - Azure-quartz, 244 - - Azurite, 287 - - - Balas-ruby, 203 - - Barnato, Barnett, 145 - - Baroque, Barrok, pearls, 292 - - Bastite, 272 - - Benitoite, 267 - - Berquem, Louis de, 90, 161 - - Beryl, 184 - - Beryllonite, 270 - - Bezel facet, 92 - - Biaxial double refraction, 45, 49, 57 - - Bisectrix, 45, 49 - - Black diamond, 129 - - Black lead, 129 - - Black opal, 249, 250 - - Black Prince’s ruby, 206 - - Blister-pearl, 296 - - Bloodstone, 247 - - Blue felspar, 255 - - Blue ground, 143, 147 - - Blue John, 285 - - Boart, 103, 129, 133 - - Bohemian garnet (pyrope), 207, 212 - - Bone turquoise, 259 - - Boodt, A. B. de, 132, 213 - - Borgis, Hortensio, 161 - - Borneo stones, 154, 170 - - Bort, _v._ Boart, 103, 129, 133 - - Bottle-stone, 284 - - Boule, 118 - - Bowenite, 263 - - Braganza diamond, 170 - - Brazil stones, 138, 165, 166, 169, 194 _et seq._, 201, 215, 223, - 236, 243, 244, 248, 266, 269, 270, 274 - - Brazilian emerald, 111, 220, 221 - peridot, 221 - sapphire, 111, 221 - topaz, 111, 197 - - Brilliant form of cutting, 92 - - Brilliant, Scientific, 122 - - Bristol diamonds, 243 - - Bruting, 100 - - Burma stones, 178, 205, 223, 227, 263 - - Button-pearl, 295 - - Byes, Bywaters, 136, 150 - - - Cabochon form of cutting, 88 - - Cacholong, 251 - - Cairngorm, 239 - - Callaica, callaina, callais, 258 - - Calcite, 40, 289 - - California stones, 156, 195, 202, 224, 259, 265, 267, 275 - - Californite, 264, 275 - - Cape-ruby, 213 - - Carat weight, 72, 84 - - Carbon, 129 - - Carbonado, 129 - - Carborundum, 105 - - Carbuncle, 89, 215 - - Carnelian, 247 - - Cascalho, 139 - - Cassiterite, 281 - - Cat’s-eye (chrysoberyl), 38, 90, 233 - (quartz), 39, 90, 240 - (tourmaline), 39, 219 - Hungarian, 244 - - Ceylon stones, 181, 195, 201, 205, 212, 215, 216, 223, 232, 236, - 237, 243, 244, 255, 267, 274, 279, 284 - - Ceylonese peridot (tourmaline), 221 - - Ceylonite, 204 - - Chalcedony, 246 - - Chatoyancy, 38 - - Chert, 247 - - Chessylite, 287 - - Chrysoberyl, 233 - - Chrysocolla, 288 - - Chrysolite (chrysoberyl), 233 - (peridot), 225 - - Chrysoprase, 247 - - Church, Sir Arthur, 61, 211, 231 - - Cinnamon-stone, 211 - - Citrine, 239 - - Cleavage, 80, 100, 149 - - Close goods, 149 - - Colenso diamond, 131 - - Colour, 53 - - Colour dispersion, 20, 97 - - Conchiolin, 293 - - Coral, 298 - - Cordierite, 266 - - Cornish diamonds, 243 - - Corundum, 172 - - Crocidolite, 39, 240 - - Crookes, Sir William, 132, 153 - - Cross facet, 93 - - Crystal, 6, 7, 8 - Rock-, 97 - - Cubic system, 8 - - Culet facet, 93 - - Cullinan diamond, 94, 100, 168 - - Culture pearls, 297 - - Cumberland diamond, 164 - - Cyanite (Kyanite), 79, 273 - - Cymophane, 234 - - - Darya-i-nor diamond, 162 - - De Beers diamonds, 167 - - Demantoid, 216 - - Density, 63 - - Deviation, Minimum, 30 - - Diamond, Characters of, 128 - cutting, 90 - gauges, 86 - Glaziers’, 135 - mining, 146 - Occurrence of, in— - Borneo, 154 - Brazil, 139 - German South-West Africa, 155 - India, 138 - New South Wales, 154 - Rhodesia, 155 - South Africa, 139 - Origin of, 151 - -point, 91 - -rose, 92 - -table, 91 - - Diamonds, Classification of, 136, 149 - Historical, 157 - Prices of, 135 - - Dichroism, 55 - - Dichroite, 266 - - Dichroscope, 55 - - Diffusion column, 65 - - Diopside, 272 - - Dioptase, 280 - - Dispersion, Colour, 20, 24, 97 - - Disthene, 273 - - Dop, 102 - - Double refraction, 28, 40 - - Doublet, 125 - - Dresden diamond, 171 - - Drop-stone, 94 - - Duke of Devonshire’s emerald, 191 - - - Edwardes ruby, 175 - - Electrical characters, 82 - - Emerald, 89, 184 - Brazilian, 220, 221 - Evening, 225 - Oriental, 111, 172 - Scientific, 122 - Uralian, 216 - - Emeraldine, 247 - - Emery, 175 - - English Dresden diamond, 166 - - Enstatite, 271 - - Epidote, 275 - - Essence d’Orient, 126 - - Essonite (Hessonite), 211 - - Euclase, 269 - - Eugénie diamond, 164 - - Evening emerald, 225 - - Excelsior diamond, 167 - - Extinction, 45 - - - Faceting machine, 105 - - False topaz, 239 - - Felspar, 254 - - Fire, 20, 96 - - Fire-marble, 289 - - Fire-opal, 251 - - Flats, 150 - - Flêches d’amour, 240 - - Flint, 247 - - Floors, 147 - - Fluor, 285 - - Frémy, E., 115 - - - Garnet, 207 - Green, 271 - - Gaudin, M. A. A., 115 - - Gauges, Diamond, 86 - - Girdle, 92 - - Glass, 7, 124 - - Gnaga Boh ruby, 180 - - Goniometer, 30 - - Grain, Pearl, 86 - - Graphite, 129 - - Greaser, 149 - - Great Mogul diamond, 161 - - Great Southern Cross group of pearls, 294 - - Great Table diamond, 162 - - Great White diamond, 167 - - Green garnet, 271 - - Greenstone, 261 - - Grossular, 211 - - - Habit, 12 - - Hardness, 78 - - Haüynite, 286 - - Heavy liquids, 64 - - Hematite, 282 - - Hessonite, 211 - - Hexagonal system, 10 - - Hiddenite, 266 - - Hope cat’s-eye, 237 - chrysolite, 237 - diamond, 170 - pearl, 294 - sapphire, 121 - - Hornstone, 247 - - Hungarian cat’s-eye, 244 - - Hyacinth, 211, 228 - - Hydrophane, 250 - - Hydrostatic weighing, 72 - - Hypersthene, 271 - - - Iceland-spar, 40, 44 - - Idocrase, 274 - - Imitation stones, 124 - - Imperial diamond, 167 - - Index of refraction, 16 - - India stones, 137, 181, 194, 215, 243, 244, 248, 290 - - Indicators, 65 - - Indicolite, 221 - - Interference of light, 39, 48 - - Iolite, 266 - - Iris, 240 - - Isle of Wight diamonds, 243 - - Isomorphous replacement, 13, 19 - - - Jacinth, 211, 228 - - Jade, 260 - - Jadeite, 262 - - Jargoon, 228 - - Jasper, 247 - - Jehan Ghir Shah diamond, 163 - - Jigger, 149 - - Jubilee diamond, 167 - - - Kauri-gum, 299 - - Khiraj-i-Alam ruby, 206 - - Kimberlite, 152 - - King topaz, 181, 201 - - Klein’s solution, 67 - - Koh-i-nor diamond, 137, 158 - - Kunz, Dr. G. F., 186, 224, 262, 265 - - Kunzite, 265 - - Kyanite, 79, 273 - - - Labradorite, 255 - - La Pellegrina pearl, 294 - - Lapis lazuli, 286 - - Lazurite, 286 - - Lozenge facet, 93 - - Lumachelle, 289 - - Lustre, 37 - - - Maacles, Macles, 12, 150 - - Madagascar stones, 195, 224, 243, 265, 266 - - Malachite, 287 - - Malacolite, 272 - - Manufactured stones, 113 - - Marble, 289 - - Mattan diamond, 155, 170 - - Matura diamonds, 232 - - Mazarin, Cardinal, 92 - - Meerschaum, 288 - - Mêlée, 136 - - Methylene iodide, 26, 66 - - Metric carat, 85, 87 - - Milky-quartz, 240 - - Minimum deviation, 30 - - Mocha-stone, 247 - - Moe’s gauge, 87 - - Mohs’s scale of hardness, 78 - - Moissan, Henri, 153 - - Moldavite, 283 - - Monoclinic system, 11 - - Moon of the Mountains diamond, 162 - - Moonstone, 39, 255 - - Morganite, 186, 195 - - Moroxite, 279 - - Moss-agate, 247 - - Mother-of-emerald, 240 - - Mother-o’-pearl, 292 - - - Nacre, 292 - - Napoleon diamond, 164 - - Nassak diamond, 163 - - Negative double refraction, 45 - - Nephrite, 261 - - Nicol’s prism, 44 - - Nizam diamond, 162 - - - Obsidian, 283 - - Occidental topaz, 111, 239 - - Odontolite, 259 - - Off-coloured diamonds, 130 - - Olivine (demantoid), 216 - (peridot), 225 - - Onyx, 247 - - Opal, 39, 249 - Fire, 251 - -matrix, 251 - - Opalescence, 39 - - Optical anomalies, 47 - - Optic axes, 49 - - Oriental almandine, 112, 172 - amethyst, 111, 172 - emerald, 111, 172 - topaz, 111, 172 - - Orient of pearls, 292 - - Orloff diamond, 160 - - Orthoclase, 254 - - Orthorhombic system, 11 - - - Pacha of Egypt diamond, 165 - - Paste, 47, 124 - - Paul I diamond, 171 - - Pavilion, 93 - - Pavilion facet, 93 - - Pear-drop pearls, 292 - - Pear-eye pearls, 292 - - Pearl, 291 - grain, 86 - imitations, 126 - - Pendeloque, 94 - - Peridot, 225 - Brazilian, 221 - Ceylonese, 221 - - Peruzzi, Vincenzio, 92 - - Phenakite, 269 - - Pigott diamond, 164 - - Pipes, 152 - - Pistacite, 275 - - Pitt diamond, 100, 159 - - Plasma, 247, 264 - - Pleochroism, 57 - - Pleonaste, 204 - - Pliny, 6, 88, 138, 184, 191, 241, 249 - - Polar Star diamond, 163 - - Polarization, 42 - - Porter-Rhodes diamond, 166 - - Positive double refraction, 45 - - Prase, 240, 247 - - Prehnite, 278 - - Pycnometer, 75 - - Pyrites, 282 - - Pyrope, 212 - - - Quartz, 50, 238 - - Quoin facet, 93 - - - Rainbow-quartz, 240 - - Reconstructed stones, 116 - - Reef, 144 - - Reflection of light, 14 - - Refraction of light, 15 - - Refractive index, 16 - - Refractometer, 22, 50 - - Regent diamond, 100, 159 - - Retgers’s salt, 69 - - Rhodes, Cecil J., 145 - - Rhodesia stones, 155, 183, 213, 236 - - Rhodolite, 62, 214 - - Rhodonite, 287 - - Rock-crystal, 97, 239 - - Rock-drill, 134 - - Röntgen rays, 83 - - Rose form of cutting, 91 - - Rose-quartz, 240 - - Rospoli sapphire, 182 - - Rotation of plane of polarization, 50 - - Rubellite, 220, 223 - - Rubicelle, 203 - - Ruby, 98, 110, 172 - Balas-, 203 - Cape-, 213 - - - Sancy diamond, 161 - - Sapphire, 98, 110, 172 - Brazilian (tourmaline), 221 - -quartz, 244 - Water- (iolite), 266 - Water- (topaz), 201 - - Sard, 247 - - Sardonyx, 247 - - Saussurite, 263 - - Schorl, 221 - - Scientific alexandrite, 122 - brilliant, 122 - emerald, 122 - topaz, 121 - - Scotch topaz, 239 - - Seed pearls, 294 - - Serpentine, 289 - - Setting of gem-stones, 107 - - Shah diamond, 163 - - Sheen, 39 - - Shepherd’s Stone diamond, 163 - - Siam stones, 180 - - Siberia and Asiatic Russia stones, 182, 188, 194, 201, 217, 223, - 236, 244, 256, 262, 269, 270, 287 - - Siberite, 221 - - Siderite, 244 - - Silver-thallium nitrate, 69 - - Skew facet, 93 - - Skill facet, 93 - - Smoky quartz, 240 - - Snell’s laws, 16 - - Soapstone, 288 - - Sodalite, 286, 287 - - Sonstadt’s solution, 67 - - South Africa stones, 139 _et seq._, 166, 167 _et seq._, 213, - 232, 244, 264, 271 - - Spanish topaz, 239 - - Specific gravity, 63 - - Specific-gravity bottle, 75 - - Spectroscope, 59 - - Spectrum, 20, 25 - - Spectrum, Absorption, 59 - - Spessartite, 216 - - Sphene, 276 - - Spinel, 203 - - Spodumene, 265 - - Spotted stones, 149 - - Star-facet, 92 - - Star of Africa diamond, 168 - - Star of Este diamond, 165 - - Star of Minas diamond, 169 - - Star of South Africa diamond, 141, 166 - - Star of the South diamond, 139, 165 - - Starstones, 38, 177 - - Steatite, 288 - - Step form of cutting, 98 - - Stewart diamond, 166 - - Strass, 124 - - Sunstone, 255 - - Synthetical stones, 113 - - Syriam, Syrian, garnet, 215 - - - Table facet, 92 - - Table form of cutting, 91 - - Tavernier, J. B., 91, 129, 137, 161, 162, 170 - - Templet facet, 92 - - Tetragonal system, 9 - - Thulite, 289 - - Tiffany diamond, 171 - - Tiger’s-eye, 39, 240 - - Timur ruby, 206 - - Titanite, 276 - - Topaz, 197 - Brazilian, 197 - False, 239 - Occidental, 111, 239 - Oriental, 111, 173 - Scientific, 121 - Scotch, 239 - Spanish, 239 - - Topazolite, 216 - - Total-reflection, 18, 21 - - Tourmaline, 43, 219 - - Trap form of cutting, 98 - - Trichroism, 57 - - Triclinic system, 12 - - Triplet, 126 - - Turquoise, 257 - - Turquoise-matrix, 258 - - Tuscany diamond, 165 - - Twinning, 12, 47 - - - Uniaxial double refraction, 45, 48 57 - - Uralian emerald, 217 - - Uvarovite, 218 - - - Variscite, 259 - - Verdite, 264 - - Verneuil, A. V. L., 116 - - Vesuvianite, 274 - - Victoria diamond, 167 - - Violane, 287 - - - Wart-pearl, 296 - - Water (of diamonds), 129 - (of pearls), 292 - - Water-chrysolite, 284 - -sapphire (iolite), 266 - -sapphire (topaz), 201 - - White opal, 249 - - White Saxon diamond, 165 - - Wollaston, W. H., 133 - - - X-rays, 83 - - - Yellow ground, 143 - - - Zircon, 228 - - - _Printed by_ MORRISON & GIBB LIMITED, _Edinburgh_ - - - +--------------------------------------------------------------------+ - | FOOTNOTES: | - | | - | [1] The word medium is employed by physicists to express any | - | substance through which light passes, and includes solids such as | - | glass, liquids such as water, and gases such as air; the nature | - | of the substance is not postulated. | - | | - | [2] Methylene iodide must be heated almost to boiling-point to | - | enable it to absorb sufficient sulphur; but caution must be | - | exercised in the operation to prevent the liquid boiling over | - | and catching fire, the resulting fumes being far from pleasant. | - | It is advisable to verify by actual observation that the liquid | - | is refractive enough not to show any shadow-edge in the field of | - | view of the refractometer. | - | | - | [3] γωνία, angle; μέτρον, measure. For details of the | - | construction, adjustment, and use of this instrument the reader | - | should refer to textbooks of mineralogy or crystallography. | - | | - | [4] A cleavage flake of topaz may conveniently be used to show | - | the phenomenon, but owing to the great width of the angle the | - | “eyes” are invisible. | - | | - | [5] In accordance with the usual custom the angle between the | - | facets is taken as that between their normals, or the supplement | - | of the salient angle. | - | | - | [6] The word paste is derived from the Italian, _pasta_, food, | - | being suggested by the soft plastic nature of the material used | - | to imitate gems. | - | | - | [7] Cf. below, p. 149. | - | | - | [8] The least and the greatest of the refractive indices of | - | doubly refractive species are given. | - | | - | [9] The dispersion is the difference of the refractive indices | - | corresponding to the B and G lines of the solar spectrum. The | - | value for crown-glass is ·016. | - | | - +--------------------------------------------------------------------+ - - - INTERESTING AND IMPORTANT BOOKS - - - JEWELLERY. By CYRIL DAVENPORT, F.S.A. With a Frontispiece in Colour - and 41 other Illustrations. Second Edition. Demy 16mo. - [_Little Books on Art._ - - JEWELLERY. By H. CLIFFORD SMITH, M.A. With 50 Plates in Collotype, - 4 in Colour, and 33 Illustrations in the text. Second Edition. - Wide royal 8vo, gilt top. [_Connoisseur’s Library._ - - GOLDSMITHS’ AND SILVERSMITHS’ WORK. By NELSON DAWSON. With 51 - Plates in Collotype, a Frontispiece in Photogravure, and - numerous Illustrations in the text. Second Edition. Wide royal - 8vo, gilt top. [_Connoisseur’s Library._ - - EUROPEAN ENAMELS. By H. H. CUNYNGHAME, C.B. With 58 Illustrations - in Collotype and Half-tone and 4 Plates in Colour. Wide royal - 8vo, gilt top. [_Connoisseur’s Library._ - - ENAMELS. By Mrs. NELSON DAWSON. With 33 Illustrations. Second - Edition. Demy 16mo. [_Little Books on Art._ - - - Transcriber’s Notes: - - Text enclosed by underscores is in italics (_italics_). - - Redundant title page has been removed. - - Blank pages have been removed. - - Front publication list moved to the back. - - Silently corrected typographical errors. - - Where possible Unicode fractions have been used, otherwise they are - formatted as example “1-5/16”. - - - - - -End of the Project Gutenberg EBook of Gem-Stones and their Distinctive -Characters, by G. F. Herbert Smith - -*** END OF THE PROJECT GUTENBERG EBOOK 60990 *** |