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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..36a3d61 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #60070 (https://www.gutenberg.org/ebooks/60070) diff --git a/old/60070-0.txt b/old/60070-0.txt deleted file mode 100644 index 845d618..0000000 --- a/old/60070-0.txt +++ /dev/null @@ -1,2937 +0,0 @@ -The Project Gutenberg EBook of Texas Gemstones, by Elbert A. King, Jr. - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Texas Gemstones - -Author: Elbert A. King, Jr. - -Release Date: August 6, 2019 [EBook #60070] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK TEXAS GEMSTONES *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - - - - - BUREAU OF ECONOMIC GEOLOGY - The University of Texas at Austin - Austin, Texas 78712 - - JOHN T. LONSDALE, _Director_ - - - Report of Investigations—No. 42 - - - - - Texas Gemstones - - - By - Elbert A. King, Jr. - - February 1961 - - _Second Printing—February 1963 - Third Printing—September 1972 - Fourth Printing—March 1983 - Fifth Printing—August 1991_ - - - - - Contents - - - Page - Introduction 5 - Properties of gemstones 5 - Crystals 7 - Cutting and polishing of gemstones 10 - Cabochon gems 10 - Faceted gems 13 - Tumbled gems 17 - Texas gemstones 18 - Amber 18 - Augite 18 - Beryl 18 - Celestite 19 - Diamond 19 - Epidote 19 - Fluorite 20 - Fossil wood 20 - Gadolinite 21 - Garnet 22 - Jet 22 - Labradorite 23 - Microcline 23 - Obsidian 24 - Opal 24 - Pearl 24 - Quartz 25 - Crystalline varieties 25 - Amethyst 25 - Citrine 25 - Rock crystal 26 - Rose quartz 26 - Smoky quartz 26 - Cryptocrystalline varieties 27 - Chalcedony 27 - Agate 27 - Agatized wood 27 - Carnelian 27 - Jasper 27 - Sanidine 28 - Spinel 28 - Tektite (bediasite) 28 - Topaz 29 - Tourmaline 30 - Turquoise 31 - Glossary 32 - Selected references 34 - Index 41 - - - - - Illustrations - - - Figures— Page - 1. Typical crystal form of three common Texas gemstones 9 - 2. Variations of the cabochon cut 10 - 3. Diamond saw 11 - 4. Cabochon properly attached to dop-stick 12 - 5. Cabochons at various stages of cutting and polishing 12 - 6. Nomenclature of the standard American brilliant cut 13 - 7. Facet table 14 - 8. Grinding the table facet on a rough stone 15 - 9. Stone dopped to table facet 15 - 10. Preformed stone dopped to table facet 16 - 11. Proper sequence of cutting of the pavilion facets 16 - 12. Proper placing of pavilion girdle facets 17 - 13. Proper sequence of cutting of crown facets 17 - 14. Common crystal form of Travis County celestite 19 - 15. Common crystal form of fluorite 20 - 16. Crystal faces on microcline specimen shown in Plate III 23 - 17. Common crystal form of spinel 28 - 18. Crystal faces on topaz crystal shown in Plate V 29 - 19. Cross section through irregularly colored stone 30 - 20. Common crystal form of Llano County tourmaline 31 - - - Plates— Page - I. A, Gem-quality celestite crystals from Travis County. B, - Opalized wood from the Texas Gulf Coastal Plain 35 - II. A, Gem-quality garnet crystals and faceted gem from Gillespie - County. B, Labradorite from Brewster County 36 - III. A, Pink microcline crystal. B, Smoky quartz. Both from Burnet - County 37 - IV. Polished agate from gravels of the Rio Grande near Zapata, - Zapata County 38 - V. A, Texas tektites (bediasites). B, Topaz crystal from a - pegmatite dike near Streeter, Mason County 39 - VI. A, Topaz from stream gravels near Streeter, Mason County. B, - Tourmaline crystals in schist from Llano County 40 - - - Table 1. Properties of some common Texas gem minerals 8 - - - - - Texas Gemstones - - - ELBERT A. KING, JR. - - - - - INTRODUCTION - - -Throughout history man has sought stones and minerals for personal -adornment and ornamentation. Stones and minerals that are sufficiently -beautiful, durable, and rare are known as gemstones. A gemstone with -only one of these qualities is less desirable than one with all three. -For example, a stone with rich color but not sufficiently durable to -withstand daily wear in rings finds little favor as a gemstone except in -brooches or pins where the stone is relatively safe from abrasion. -Likewise, a stone that is beautiful and durable may be of little -interest as a gemstone because it is commonly found in great quantities. -To be valued highly, gemstones must be beautiful to the eye, durable -enough to withstand wear, and rare enough so that they are not easily -obtained. - - - - - Properties of Gemstones - - -The beauty of gemstones is mostly dependent on their color, diaphaneity, -brilliancy, luster, and fire. Any one or a combination of these -properties render stones desirable as gems. - -Color is very important in many gemstones. The color of transparent -varieties should be distinct enough to be pleasing to the eye, yet not -so dark as to appear black or opaque. It is generally more desirable -that the gemstone be of even color and not appear “patchy” or -“streaked.” However, some opaque or translucent stones such as agate owe -their popularity chiefly to the variety of colors and designs within a -single piece. Some transparent gemstones exhibit different colors when -viewed in different directions. For example, some fine blood-red rubies -appear brownish when viewed in a particular direction. The gemstone -should be cut so that its finest color is most prominently displayed. -This ability of some gemstones to exhibit different colors when viewed -in different directions is called pleochroism. - -Diaphaneity is the relative ability of stones to transmit light. -Diaphaneity is described by terms such as transparent, translucent, and -opaque. Transparency is highly desirable in stones such as diamond that -are commonly facet-cut to reflect light. The gemstone should be water -clear and free from inclusions and cracks so that it transmits light -freely, but there are stones that do not exhibit this property that are -prized as gemstones. For example, turquoise may appear to be completely -opaque and not transmit any light, but it is sought for its fine blue -color. - -The brilliancy of gemstones is largely dependent on their index of -refraction. The index of refraction is a measure of the ability of a cut -gemstone to “bend” light rays and reflect them from the bottom facets -back through the top of the stone. Of course, brilliancy is not noted in -opaque or faintly translucent stones. The index of refraction of -gemstones is expressed numerically. Air is the reference medium and is -assigned an index of refraction of 1.00. Other substances are assigned -values relative to that of air, for example, water, 1.33; topaz, 1.62; -diamond, 2.42. The higher the index of refraction, the more brilliant -will be the gemstone if it is properly cut and polished. - -Luster is the appearance of the mineral on a fresh surface in reflected -light; it is divided into two major categories, metallic and -non-metallic. Most gemstones have non-metallic luster and are described -by terms such as vitreous or glassy, resinous, waxy, greasy, and pearly. - -The fire, or ability of gemstones to show flashes of different colors of -light, is dependent upon a property called dispersion. The amount of -dispersion is the extent to which the gemstone is able to separate -ordinary white light into its component colors. The dispersion of -gemstones can also be expressed numerically but for purposes of this -publication will be referred to as low, moderate, or high. Diamond is a -common gemstone that has high dispersion. - -A gemstone’s durability is primarily dependent upon its hardness. The -Mohs scale of hardness, given below, is most commonly used for gemstones -and other minerals. - - _Mohs Scale of Hardness_ - 1. Talc - 2. Gypsum - 3. Calcite - 4. Fluorite - 5. Apatite - 6. Orthoclase feldspar - 7. Quartz - 8. Topaz - 9. Corundum - 10. Diamond - -On this scale, the higher numbers are the harder minerals. Mohs is a -relative, not an absolute scale. Therefore, it should not be assumed -that diamond is ten times harder than talc because actually diamond is -very many tens of times harder than talc. However, a particular mineral -is harder than any other mineral with a lesser number, and the scale is -very convenient to use. Gemstones mounted in rings should have a -hardness of at least seven on the Mohs scale, or the stones may become -scuffed and scratched after a relatively short period of wear. Gemstones -mounted in pins and brooches can be of softer material as they are not -usually subjected to abrasion and rough treatment. - -The tendency of some minerals to split with relative ease in particular -directions along planes is called cleavage. Cleavage is also a factor -determining the durability of gemstones. Some gemstones do not exhibit -this tendency at all, whereas others cleave in several directions. The -number of cleavages is always the same in any one mineral, and the -direction of cleavages is constant in relation to the crystal structure -of any one mineral or gemstone. It is apparent that of stones having the -same hardness, the ones lacking cleavage or having the lesser number of -good cleavage directions are the most durable. - -Some stones, such as jade and agate, owe their durability to their -compact fibrous structure, which makes them very tough and durable even -though they are not especially hard. - -Several other properties of gemstones, although not always contributing -to the beauty or desirability of gemstones, are useful in identifying -uncut specimens. - -Streak is the color of the mineral when finely powdered or, for softer -minerals, the color obtained by rubbing the mineral against a piece of -unglazed porcelain or tile. The color of a mineral’s streak is commonly -different from the unpowdered specimen. - -Fracture is the kind of surface obtained when the mineral is broken in a -direction that is not a cleavage direction. Fracture surfaces are -described by such terms as conchoidal (like the fracture of glass), -subconchoidal, splintery, even, and uneven. - -Tenacity is the resistance of a mineral to breakage. Brittle minerals -break relatively easily on impact. Malleable minerals, such as gold, may -be flattened under a hammer into very thin sheets without breaking. -Sectile minerals may be cut with a knife without powdering. Most -gemstones, even diamond, are brittle. - -It is only natural to value most those gemstones that are not common or -easy to obtain. Emerald owes its longstanding popularity to its fine -green color, but tourmaline is sometimes found in colors that very -closely approach that of emerald and yet sells for considerably less -because it is so much more common. - -Rarity is not the only factor affecting the value of gemstones. Freedom -from internal imperfections, quality of cutting, color, and size must -also be considered in cut and polished gemstones. Internal -imperfections, such as inclusions and cracks, detract from the -appearance of gemstones and interfere with the passage of light between -the facets; consequently, gemstones containing these imperfections are -not valued as highly as those without them. Poor cutting or polishing -detract from the beauty and thus from the value of gemstones. Unpopular -or poor color commonly causes gemstones to be less valuable. Rich green -emeralds are exceedingly prized, whereas very pale green emeralds are -relatively inexpensive. Diamonds that have the least hint of yellow are -never valued as highly as pure colorless, pink, or blue stones. Few -persons find the yellowish color attractive, unless it is a vivid canary -yellow. - -Size is important in determining the value of gemstones but not as -important as perfection. A badly flawed gemstone of large size may be -worth only a slight fraction of the value of a smaller perfect one. -Gemstone size is usually measured in carats, a unit of weight, although -millimeter size is sometimes used. Five carats is equal to 1 gram and -approximately 28⅓ grams is equal to 1 ounce avoirdupois. One -one-hundredth (0.01) of a carat is called a point, and this term is -often used, especially pertaining to very small gemstones. - -The term used to compare the relative weights of minerals and gemstones -is specific gravity, which is expressed numerically in relation to -water. Water is assigned the value of 1.00. Therefore, at a given -temperature a gemstone having a specific gravity of 2.00 is twice as -heavy as an equal volume of water. A 1-carat sapphire (specific gravity -about 4.00) will be smaller than a 1-carat amethyst (specific gravity -about 2.65) because the heavier material will occupy less volume to have -the same weight. - -A summary of properties helpful in identification of common Texas gem -minerals is given in Table 1. - -Comparatively recently in the history of gemstones, man has succeeded in -the production of synthetic gems that have properties closely -approaching those of many natural gemstones. To the untrained eye some -synthetic gems may appear identical to natural stones, but synthetic -gems can be detected with little difficulty by a properly equipped -expert. Although most synthetic gems are inexpensive, their manufacture -has not adversely affected the value of natural gemstones but instead -has increased the demand for fine natural gems. - - - - - Crystals - - -Gemstones that have an orderly internal molecular arrangement are -referred to as crystalline. This internal order is commonly reflected in -the external shape of “rough” or uncut gemstones. The resultant shape is -a polyhedral solid bounded by planes and called a crystal. Well-formed -crystals are formed in nature only under relatively ideal conditions of -temperature, pressure, and space. The specific temperatures and -pressures involved vary with different minerals, but most crystals need -space in which to form so that their “growth” is not impaired by -surrounding rocks and minerals. However, some minerals, such as garnet -and tourmaline, can grow in metamorphic rocks by recrystallization of -minerals in the metamorphic rocks. The size of crystals varies from -microscopic to tens of feet. Any one mineral usually has one or two -typical crystal forms or arrangements of plane surfaces that aid greatly -in the identification of the mineral when it occurs in good crystals -(fig. 1). Frequently gemstones are found as abraded stream-rolled -pebbles, fragments, or masses that do not show crystal form. Crystals of -the same mineral from different locations commonly show somewhat -different crystal forms owing to slight differences in composition or -conditions of formation. Mineralogists and crystallographers classify -crystals by the symmetry that they exhibit. The crystal systems are (1) -isometric or cubic, (2) tetragonal, (3) hexagonal, (4) orthorhombic, (5) -monoclinic, and (6) triclinic. A complete description of the -classification of crystals can be found in almost any mineralogy text -(see Selected References, p. 34). - - Table 1. Properties of some common Texas gem minerals. - MINERAL COMPOSITION HARDNESS SPECIFIC INDEX OF COMMON - GRAVITY REFRACTION COLORS IN - TEXAS - - Amber fossil resin 2.0-2.5 1.05-1.10 about 1.54 brown, yellow - Augite CaMgSi₂O₆ 5.0-6.0 3.2-3.6 1.60-1.71 greenish - brown, black - Beryl Be₃Al₂(SiO)₆ 7.5-8.0 2.63-2.80 1.56-1.60 pale blue, - colorless, - greenish - Celestite SrSO₄ 3.0-3.5 3.95-3.98 1.62-1.63 colorless, - blue - Epidote HCa₂(Al, 6.0-7.0 3.25-3.50 1.72-1.77 yellowish - Fe)₃Si₃O₁₃ green, - brownish - green - Fluorite CaF₂ 4.0 3.0-3.25 1.434 colorless, - violet, - yellow, green - Garnet Fe₃Al₂(SiO₄)₃ about 7.5 4.25 about 1.83 red, deep - (Almandite) red, - brownish red - Labradorite NaAlSi₃O₈ 50% to 6.0-6.5 about 2.6 about 1.56 yellowish, - 30% CaAlSi₃O₈ 50% grayish - to 70% - Microcline KAlSi₃O₈ 6.0-6.5 2.54-2.57 1.52-1.53 pink, red, - bluish, - greenish, - white - Obsidian volcanic glass 5.0-5.5 2.3-2.5 1.45-1.53 dark gray, - black, - brownish - Opal SiO₂·nH₂O 5.5-6.5 1.9-2.3 1.43 white, pink, - bluish, - brown, gray - Quartz SiO₂ 7.0 2.65-2.66 1.544-1.553 colorless, - (Crystalline) violet, - yellow, brown - Tektite natural glass 5-6 2.33-2.44 1.48-1.52 dark brown, - (Bediasite) greenish - brown - Topaz Al₂(F·OH)₂SiO₄ 8.0 3.4-3.6 1.60-1.63 colorless, - bluish, sky - blue - Tourmaline H₉Al₃(B·OH)₂Si₄O₁₉ 7.0-7.5 2.98-3.20 1.62-1.64 black, dark - brown - -Some gemstones, such as opal and obsidian, never occur as crystals owing -to a lack of internal structural order. Such gemstones are termed -amorphous, or without form. Amorphous gemstones mostly occur in nature -as irregular lumps or masses, cavity fillings, or veins. - - [Illustration: Fig. 1. Typical crystal form of three common Texas - gemstones.] - - GARNET - TOURMALINE - QUARTZ - - - - - CUTTING AND POLISHING OF GEMSTONES - - -There are two types of widely used gemstone cuts. Opaque or figured -gemstones are usually cut with a rounded upper surface and a flat or -rounded back. A stone cut in this fashion is termed a cabochon or is -said to be cabochon cut. There are several variations of this mode of -cutting (fig. 2). Precious opal, agate, jade, star sapphire, and fossil -wood are some of the stones that are cut mostly as cabochons. -Transparent gemstones are usually cut with many plane polished surfaces. -Such stones are called faceted, and the process of cutting and polishing -these stones is called faceting. Emerald, diamond, topaz, and garnet are -examples of gemstones that are commonly seen as faceted stones. - - [Illustration: Fig. 2. Variations of the cabochon cut. Left to right: - double cabochon; flat cabochon; simple cabochon; hollow cabochon.] - -The cutting of gemstones, although sometimes tedious and time consuming, -is not especially difficult or complex. However, like most arts and -crafts, technique and ability should improve with practice and -experience. There are currently many amateur gem cutters in Texas. A -complete set of equipment necessary to cut cabochon stones may be -purchased for as little as $50.00 or $60.00. Most amateur cabochon -cutters have equipment that cost less than $100.00 which enables them to -do very fine work on many gem materials. Facet cutting requires more -precise equipment, and a complete array of such usually costs more than -$100.00, although less expensive equipment can be obtained. The -beginning gem cutter or lapidary who is willing to assemble and make -some of his own equipment can reduce his initial expenses considerably. - - - - - Cabochon Gems - - -The procedures listed herein for gem cutting do not apply to all -gemstones. Stones that are especially brittle, soft, or difficult to -polish require additional procedures or special techniques. Many -lapidaries may deviate from these procedures. Some of the steps of -cutting and polishing are merely matters of personal opinion and vary -somewhat from cutter to cutter. There are several detailed texts on the -art of gem cutting; the descriptions herein are designed to give the -reader only a general idea of the procedures and techniques involved. - -The cutting and polishing of cabochons require several steps. The -initial step is sawing. Assuming that the rough gem material is large -enough to be sawed (larger than about half an inch in diameter), it is -clamped into the carriage of a diamond saw (fig. 3) and cut into slices -about ⅜-inch thick. The blade of the saw is mild steel that has been -impregnated with diamond dust around the edge, hence the name diamond -saw. The blade is rotated rapidly, and the material to be cut is “fed” -to the blade by a sliding carriage on which the gem material is clamped. -The extreme hardness of the diamond dust in the edge of the blade -enables the saw to cut through several inches of gem material in a few -minutes. The lower portion of the saw blade is immersed in a mixture of -kerosene and oil, and the rotating saw blade carries with it some of the -kerosene-oil mixture; this acts as a coolant and lubricant for both the -saw blade and the material being cut. Without this lubricant, the heat -generated by sawing would shatter most gem materials and also damage the -saw blade. As this “slicing” or sawing of the material usually takes -several minutes, a weight and pulley are generally used to give the gem -material the necessary pressure against the saw blade. When cut through, -the “slab” of gem material falls into the kerosene-oil mixture at the -bottom of the saw or onto a special platform that cushions its fall. - - [Illustration: Fig. 3. Diamond saw.] - - Motor - Clamp - Diamond-charged blade - Carriage - Stone - Weight - -After being sawed, the slab of gem material is examined, and the -location and size of the stones to be cut from the slab are determined. -The desired outline of the shape of the gem to be cut is marked on the -slab with a pointed piece of aluminum rod; ordinary pencil marks are not -used because they wear away too quickly in the cutting process. Once the -area from which the gem is to be cut has been selected and the outline -of the gemstone has been marked on the slab, the excess material is -trimmed away by a smaller diamond saw known as a trim-saw. In some slabs -the excess material can be broken and “nibbled” away with a strong pair -of pliers. - -The remaining portion of the stone is usually held by hand and ground to -the desired shape using the previously scribed mark as a guide. This is -done using a relatively coarse-grained (about 150 grit) specially made -carborundum grinding wheel. - -Now that the desired outline has been obtained, the stone is firmly -affixed to a slender wooden or hollow aluminum dop-stick (fig. 4). The -process whereby the stone is attached to the dop-stick with a specially -compounded jeweler’s wax is called dopping. The dop-wax is heated over -an alcohol lamp or candle flame until it is soft and pliable and is then -spread around on the end of the dop-stick and formed into a mass about -the right size and shape to fit the back of the gemstone. The stone is -likewise heated, and the wax is applied to the back of the stone while -both wax and stone are hot. Upon cooling, the wax firmly fixes the stone -to the dop-stick. The dop-stick allows the lapidary to have firm control -of the stone during all later stages of cutting and polishing. - - [Illustration: Fig. 4. Cabochon properly attached to dop-stick.] - - CABOCHON - DOP-WAX - DOP-STICK - -The top of the dopped gemstone is worked against the coarse carborundum -grinding wheel until it is a rough approximation of the desired shape. -The stone is then worked against a much finer-grained (about 220 grit) -grinding wheel to remove the irregularities left by the coarse grinding -and to further smooth and shape the surface of the gemstone. At all -times while grinding, a small flow of water should be directed on the -grinding wheel to keep the stone cool. Grinding on the stone for even a -few minutes without cooling may result in the shattering of the gemstone -because of heat created by friction of the stone against the grinding -wheel. If the lapidary keeps the surface of the grinding wheel wet, -there is little chance of damaging most gem materials. - -The next phase of cabochon cutting and polishing is sanding. The -gemstone is worked against two sanding drums of different grit size. -This sanding can be done with the sandpaper surface either wet or dry, -as needed or as preferred by the lapidary. However, great care should be -exercised during sanding so that the stone is not overheated. -Overheating can easily occur whether the sandpaper is used wet or dry. -As in grinding, sanding is first done on coarser grit paper (about 300 -grit) and last on finer paper (about 600 grit). It is in the sanding -process that the first hint of polish is noted on the surface of the -stone. After sanding, the gemstone should have perfect form with no -surface irregularities, a very finely textured surface, and only very -minor scratches left from sanding. The gemstone is now ready to be -polished. - - [Illustration: Fig. 5. Cabochons at various stages of cutting and - polishing. Left to right: trimmed from slab: ground to outline; after - rough grinding; after sanding; polished.] - -At this point the procedure depends on the nature of the gemstone being -polished. Most gem materials are worked against a buffing wheel that is -impregnated or saturated with a mixture of some polishing compound and -water. A soft felt buffing wheel with cerium oxide as the polishing -agent is used for many materials. The mixture of cerium oxide and water -is usually applied to the buffing wheel with a small brush. The lapidary -should once more be careful not to overheat the stone. If the stone -becomes too hot to hold to the underside of the cutter’s wrist, it -should be permitted to cool for a few seconds before continuing. After -polishing on the buffing wheel, the gemstone should have a fine, high -polish and be free of any scratches or surface irregularities. The -finished gemstone is removed from the dop-stick by heating the dop-wax -and pulling the stone loose. Any excess wax that hardens again before it -can be removed from the stone by hand can be dissolved away by rubbing -with an acetone-soaked cloth. Figure 5 illustrates the desired -appearance of the gemstone at the end of each of the steps of cutting -and polishing. - - - - - Faceted Gems - - -The principles involved in faceting are about the same as those in the -cutting of cabochons, but the equipment and technique are considerably -different. The equipment required for the facet cutting of gemstones is -built into or attached to a small specially constructed table (fig. 7), -and the unit is commonly called a facet table. Most faceted gemstones -are cut to obtain the largest flawless stone possible from the rough -material. Therefore, one of the first and most important steps for the -lapidary is to decide how the stone is to be cut from the rough crystal -or pebble. The colors that can be obtained from the gemstone must also -be considered, and the cutting of the stone oriented so that its best -color is displayed. The lapidary also selects the orientation of the -stone in relation to the cleavage or cleavages. It is difficult or -impossible to polish facets of gemstones that are cut parallel to a good -cleavage direction. - - [Illustration: Fig. 6. Nomenclature of the standard American brilliant - cut.] - - TOP VIEW - SIDE VIEW - Star facet - Crown main facet - Crown girdle facet - Pavilion girdle facet - Pavilion main facet - TABLE - CROWN GIRDLE - PAVILION - CULET - BOTTOM VIEW - -Once the orientation of the gemstone to be cut from the rough material -has been determined, the stone is dopped onto a special metal dop-stick -that fits into the chuck of the facet head. The chuck is tightened so -that the position of the stone on the end of the arm of the facet head -is firmly fixed, and the facet head is adjusted so that the first facet -that is cut is the horizontal, top facet of the stone or table facet -(fig. 6). The table facet is cut by grinding the gemstone on a flat -cutting lap that is diamond impregnated (fig. 8). By minor adjustments -of the facet head, the lapidary can precisely control the location of -the table facet. As soon as the table facet has been ground to the -proper size, the cutting lap is removed from the lap plate, and the -polishing lap is secured in place. Many different kinds of polishing -laps and polishing compounds may be used depending on the properties of -the material being polished. However, one lap and one polishing compound -are usually sufficient for each gem variety. After the polishing lap is -secured to the lap plate, the lapidary adjusts the facet head so that -the stone is in exactly the same position relative to the lap that it -was during the cutting of the table facet. The polishing lap is run wet -or damp with water, as is the cutting lap, and small amounts of the -polishing compound are applied to the surface of the lap while the facet -is being polished. The minor scratches left by the cutting process are -gradually removed, and a fine lustrous polish develops on the facet. It -is especially important to take care in achieving a perfect polish on -the table facet, as this facet occupies a large area of the crown of the -gemstone. When the cutting and polishing of the table facet are -completed, the gemstone is still rough or uncut in all portions except -for this single, large, polished surface. - - [Illustration: Fig. 7. Facet table.] - - Water - Light - Adjusting ring - Post - Arm - Chuck - Stone - Abrasives - DIAMOND DUST - CALCIUM OXIDE - LANDE-A - - [Illustration: Fig. 8. Grinding the table facet on a rough stone.] - - CHUCK - DOP-STICK - DOP-WAX - STONE - LAP - -The gemstone is then removed from the dop-stick by melting the dop-wax -and is dopped once more so that the plane of the polished table facet is -perpendicular to the axis of the chuck and arm of the facet head (fig. -9). Great care should be taken by the lapidary to insure that the table -of the stone is exactly perpendicular to this axis, or the proper -placing of the later facets on the stone may become very difficult. - - [Illustration: Fig. 9. Stone dopped to table facet.] - - TABLE FACET - DOP-WAX - STONE - DOP-STICK - -Once the stone has been properly dopped to the table facet, the lapidary -is ready to proceed with the cutting of the outline of the stone. If it -is to be a brilliant cut, the stone is ground perfectly round in -outline; if it is to be an emerald or step cut, it is shaped so that it -is square or rectangular in outline. This process is called preforming. -The arm of the facet head is lowered on the post until it is horizontal, -and the stone is worked against the cutting lap until the desired shape -is obtained. When the preforming process is completed, the stone should -have the desired outline of the finished gem (fig. 10). - - [Illustration: Fig. 10. Preformed stone dopped to table facet.] - - DOP-WAX - STONE - DOP-STICK - -The lapidary is now ready to proceed with the cutting of the pavilion of -the stone. The arm of the facet head is raised to the proper angle for -cutting the main pavilion facets. The angle at which the main facets are -cut is very critical in determining the beauty of the finished stone. -The required angle at which these facets must be cut varies with the -refractive indices of the different varieties of gem minerals. If the -facets are not cut at exactly the proper angle, light entering the top -or crown of the gemstone can pass completely through the stone, instead -of being reflected back out of the crown facets. The result is a dull, -lifeless stone that appears to have a “hole” or “fish-eye” in the -center. Stones that are cut in this manner are greatly reduced in value. -The angle at which the facets are cut is controlled by the adjustment of -the height of the arm of the facet head on the post. The lapidary will -continually adjust this height, because the angle between the arm and -the surface of the lap changes slightly as the facet is ground down to -its proper place and size. - - [Illustration: Fig. 11. Proper sequence of cutting of the pavilion -facets. Left to right: four main facets; all eight main facets; half of - the pavilion girdle facets; completed pavilion.] - -The standard American brilliant cut will be used as an example of facet -cutting. Procedure for all other cuts is essentially the same to this -point. After the eight main pavilion facets have been cut, the cutting -angle is changed a few degrees, the arm of the facet head rotated -slightly, and the sixteen pavilion girdle facets or “skill” facets, as -they are often called, are cut (fig. 11). The pavilion girdle facets -should meet exactly in the center of the main facets at the girdle of -the stone. The pavilion girdle facets should neither overlap, nor should -there be any space between them (fig. 12). After the pavilion girdle -facets are cut, the cutting of the pavilion of the gemstone is -completed. The facets are then polished on the polishing lap at the same -angles and in the same order as they were cut, and the pavilion of the -gem is completely finished. - -The stone is then removed from the dop-stick by melting the dop-wax and -is re-dopped to the pavilion facets so that the crown of the stone is -now exposed for cutting. Before the lapidary proceeds with the cutting -of the crown, it is necessary that the stone be perfectly centered on -the dop-stick and that the plane of the table facet be perpendicular to -the dop-stick and to the axis of the arm of the facet head. The eight -main facets are cut first, with numerous adjustments being made by the -lapidary to insure that the proper angle is maintained (fig. 13). Then -the cutting angle is changed a few degrees, the arm of the facet head -rotated slightly, and the crown girdle facets are cut. The crown girdle -facets are placed very similarly to the pavilion girdle facets except -that they are shorter. The crown girdle facets should be joined in -exactly the same way as the pavilion girdle facets. When these facets -are properly cut, the cutting angle is again changed, the arm rotated, -and the eight star facets are cut. This completes the cutting of the -crown of the stone. The cutting lap is removed from the lap plate, and -the polishing lap is secured into place. The facets are carefully -polished in the same order that they were cut. After the last star facet -has been polished, the stone is removed from the dop-stick. Any excess -dop-wax is removed from the stone by means of a solvent, and the full -beauty of the finished gem is revealed. - - - - - Tumbled Gems - - -One other method of finishing gemstones that deserves mention is -tumbling. “Baroque” or “free-form” stones are produced in this manner. -Loose pebbles or pieces of gem materials left over from other cutting -processes are placed in a small barrel or specially constructed box with -loose carborundum grit. The barrel is turned by means of a small motor, -and the abrasion of the pebbles and grit against each other tends to -round the pebbles and give them a finely pitted surface. Progressively -finer and finer carborundum grit is used, and eventually a polishing -compound. The result is several pounds of well-polished gem pebbles of -various shapes and sizes. These baroque stones have found recent favor -in costume jewelry of modern design. The tumbling process is rather -slow, commonly requiring several days or weeks. However, little effort -is involved on the part of the lapidary, and, consequently, the cost of -most tumbled or baroque stones is quite modest. Only gem material that -is unsuitable for cutting in other manners should be finished in this -way. - - [Illustration: Fig. 12. Proper placing of the pavilion girdle facets. - Left: facets not joined. Center: facets overlapped, joined too high. - Right: correct placing.] - - Stone - Dop-wax - Dop-stick - Chuck - -[Illustration: Fig. 13. Proper sequence of cutting of the crown facets. - Left to right: four main facets; all eight main facets; half of the - crown girdle facets; completed crown.] - - - - - TEXAS GEMSTONES - - - - - Amber - - - _Composition_: fossil resin. _Crystal system_: amorphous. _Hardness_: - about 2.0 to 2.5. _Specific gravity_: variable, from 1.05 to 1.10. - _Luster_: resinous. _Color_: brown, yellow, red, orange, and white. - _Streak_: white to yellowish to gray. _Cleavage_: none. _Fracture_: - conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to - translucent. _Refractive index_: variable, about 1.54. Burns with a - sweet, piney odor. - -Rich brown to yellowish amber has been found near Eagle Pass, Maverick -County, in Cretaceous coal and on Terlingua Creek, Brewster County. -Although much of this material is translucent and the quality suitable -for lapidary purposes, the pieces are seldom more than a fraction of an -inch in diameter. - -Occasional finds of poor quality brownish amber have been reported from -the Tertiary formations of the Gulf Coastal Plain, but thus far no gem -quality material has been found. - -The softness of amber limits its use to brooches, necklaces, and other -jewelry that is relatively safe from abrasion. - - - - - Augite - - - _Composition_: CaMgSi₂O₄; may also contain iron, aluminum, and - sometimes titanium. _Crystal system_: monoclinic. _Hardness_: 5 to 6. - _Specific gravity_: 3.2 to 3.6. _Luster_: vitreous to dull. _Color_: - dark greenish brown and greenish black. _Streak_: light grayish green. - _Cleavage_: two directions, poor. _Fracture_: conchoidal to uneven. - _Tenacity_: brittle. _Diaphaneity_: opaque to translucent. _Refractive - index_: variable, about 1.60 to 1.71. - -Augite of gem quality occurs near Eagle Flat, Hudspeth County, Texas. -Although this material is very dark greenish brown and not commonly -thought of as a gemstone, lapidaries have used it to fashion black -faceted stones and cabochons that resemble obsidian. Most of the augite -occurs as loose pieces and crystal fragments that have weathered out of -nearby igneous rocks; the augite can also be found in situ in the -igneous rocks. - -Specimens and pieces of cutting quality 1 inch in diameter are common, -and fragments over 2 inches in diameter have been found. The augite is -associated with black spinel and some dark gray to black pieces of -natural glass. Although the faceted and cabochon-cut stones are not -particularly attractive, some of the larger pieces of augite might be -utilized for carving. - - - - - Beryl - - - _Composition_: Be₃Al₂(SiO)₆. _Crystal system_: hexagonal. _Hardness_: - 7.5 to 8.0. _Specific gravity_: 2.63 to 2.80. _Luster_: vitreous. - _Color_: pale blue, blue, green, yellow, brownish, pink, and - colorless. _Streak_: white. _Cleavage_: one direction, very imperfect. - _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: - transparent to subtranslucent. _Refractive index_: 1.56 to 1.60. - _Dispersion_: low. - -Gem-quality beryl has not been reported in Texas. A discussion of beryl -is included herein because the writer believes it likely that beryl of -gem quality will be found in Texas as a result of future investigations -and exploration. - -Beryl crystals have been found in pegmatite dikes in Llano, Blanco, and -Gillespie counties. These crystals are commonly several inches long and -exceed 1 inch in diameter but are very badly fractured. Most of the -beryl crystals do not approach gem quality and are entirely unsuitable -for any lapidary use. The color of the crystals found thus far is -bluish, greenish, pinkish brown, yellowish, and colorless. Some very -tiny colorless beryl crystals have been found that are transparent, but -thus far such crystals have been too small to be cut into gems. - -Fine blue beryl crystals have been found in the Franklin Mountains near -El Paso, Texas. Unfortunately, these crystals are so badly flawed and -fractured that they are not suitable for lapidary use. - -It seems likely that careful prospecting of Texas pegmatites will reveal -at least some gem-quality beryl. - - - - - Celestite - - - _Composition_: SrSO₄. _Crystal system_: orthorhombic. _Hardness_: 3.0 - to 3.5. _Specific gravity_: 3.95 to 3.98. _Luster_: vitreous. _Color_: - white, blue, greenish, reddish, and brownish. _Streak_: white. - _Cleavage_: three directions, although one of these directions is not - easily developed. _Fracture_: uneven. _Tenacity_: brittle. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: 1.62 - to 1.63. _Dispersion_: moderate. - -Celestite is very seldom cut into gems. Being very soft, brittle, and -having three cleavages, celestite is completely unsuitable for jewelry. -These same properties make this mineral exceedingly difficult to facet; -however, faceted stones are seen in large collections. - -[Illustration: Fig. 14. Common crystal form of Travis County celestite. - Same crystal form as shown in Plate I, A.] - -Fine crystals of colorless and blue gem-quality celestite (Pl. I, A, and -fig. 14) have been found at Mount Bonnell and other localities west of -Austin, Travis County. The celestite crystals occur in vugs or geodes in -limestone. The crystals are mostly white or colorless and fractured near -the base or where attached, but the tips of the crystals are commonly -clear celestine blue and completely free of flaws. - -Crystals several inches in length have been found, but the average size -is about 1 inch. The smaller crystals are frequently more transparent -and consequently better suited for cutting. It is very difficult to -obtain crystals that will allow the cutting of flawless stones of more -than 4 or 5 carats. - -Bluish and colorless celestite of gem quality and fine crystals have -been found near Lampasas, Lampasas County, and near Brownwood, Brown -County, but neither of these localities has been very productive of good -gem material. - -Celestite geodes have been found in parts of Coke, Fisher, and Nolan -counties, but these geodes contain little gem material. - - - - - Diamond - - - _Composition_: carbon. _Crystal system_: isometric. _Hardness_: 10. - _Specific gravity_: 3.51 to 3.53. _Luster_: adamantine to greasy. - _Color_: brown, colorless, pink, blue, yellow, and various other light - colors; rarely deeply colored; sometimes black. _Cleavage_: four - directions, octahedral, perfect. _Fracture_: conchoidal. _Tenacity_: - brittle. _Diaphaneity_: transparent to opaque. _Refractive index_: - 2.42. _Dispersion_: high. - -There is only one well-authenticated find of diamond in Texas. A small -brownish diamond was found in 1911 on section 64, block 44, Foard County -(Sterrett, 1912, pp. 1040-1041). The exact weight of the stone has not -been recorded, but one authority estimated that it was of sufficient -size and clarity to yield a cut stone of about one-quarter carat. - -The only diamond-bearing rocks known in the United States are in Pike -County, Arkansas. Although many other diamonds have been found in the -United States, all were loose in gravels or streams except for some -stones at the Arkansas locality. The fact that one diamond was found in -Foard County does not mean that the prospects of finding more diamonds -in Texas are much better there than anywhere else in the State. It is -highly unlikely that more than a very few diamonds will ever be found in -Texas, and any stones that may be found in the future are likely to be -widely scattered. - - - - - Epidote - - - _Composition_: HCa₂(Al, Fe)₂Si₃O₁₃. _Crystal system_: monoclinic. - _Hardness_: 6 to 7. _Specific gravity_: 3.25 to 3.5. _Luster_: - vitreous. _Color_: yellowish green to brownish green and brown. - _Streak_: uncolored to grayish. _Cleavage_: two directions. - _Fracture_: uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to - opaque. _Refractive index_: about 1.72 to 1.77. - -Llano County has furnished some green and brownish-green epidote that is -suitable for cutting into cabochons. Most of the material that -approaches gem quality has come from contact metamorphic zones and is -associated with garnet, quartz, and some scheelite. Some small cavities -in the rocks contain tiny transparent crystals of gem quality, but the -largest obtainable flawless faceted stones would probably be less than -15 points. - -Faceted stones of epidote are sometimes known as pistacite owing to -their common pistachio-green color. - - - - - Fluorite - - - _Composition_: CaF₂. _Crystal system_: isometric. _Hardness_: 4. - _Specific gravity_: 3.0 to 3.25. _Luster_: vitreous. _Color_: violet, - blue, colorless, green, yellow, brown, rose, and crimson red. - _Streak_: white. _Cleavage_: four directions, octahedral, perfect. - _Fracture_: subconchoidal to splintery. _Tenacity_: brittle. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: - 1.434. - -Very fine green, transparent fluorite has been found near Voca, Mason -County. The fluorite occurs as vug fillings in pegmatites, associated -with crystals of pink microcline and colorless quartz. Most of the vugs -have been completely filled by the fluorite; therefore, crystals (fig. -15) of the fluorite are not too common. Masses of fluorite several -pounds in weight, rich green, and quite transparent have been found near -Voca. Transparent pieces an inch or more in diameter are common. - - [Illustration: Fig. 15. Common crystal form of fluorite.] - -Fluorite is much too soft for everyday use in jewelry and because of the -low refractive index does not yield brilliant faceted stones. The -perfect four-directional cleavage, relative softness, and brittle -tenacity of the mineral make it difficult to facet. Faceted stones are -seldom seen outside of collections. Cabochons are also difficult to cut -from this material, but the rich color obtained is ample reward for the -time and care necessary in cutting. - -Fluorite occurs at several other localities in Texas, notably in -Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not -commonly in gem quality or colors that warrant its use as gem material. - - - - - Fossil Wood - - -Wood that is buried in silica-rich sediments is commonly replaced by -quartz, agate, or opal. The wood structure, including a large number of -the annular rings, knots, small branches, and bark, may be preserved. -This process of replacement by silica is believed to take considerable -time. Preservations by other means (_see_ Jet, p. 22) are known, but -silica replacements are most commonly used as gem materials. - -Fossil wood is often used by lapidaries as gem material when mineral -replacement preserves the wood structure sufficiently well and when -various impurities color the replacement material attractively. - -Excellent gem-quality fossil wood (Pl. I, B) has been found at a great -number of localities in Texas. Agatized and opalized wood occurs in -great abundance along the outcrops of Eocene and Oligocene strata of the -Texas Gulf Coastal Plain. Much of this material is very well suited for -cabochons, bookends, and other lapidary uses. The preservation is -especially good at numerous localities in Washington, Lee, Fayette, and -Gonzales counties, and the variety of colors, such as bluish, gray, -brown, red, yellow, and black, makes this material especially sought -after by “rock-hounds.” Some of the agatized and opalized wood -fluoresces yellow or green under ultra-violet light. The fossil wood is -sometimes found as stumps, limb sections, or large trunk fragments, but -the great majority of the gem material is found as small broken -fragments or stream-rolled cobbles. - -Fossil palm wood is by far the most sought after variety because this -material displays “eyes” and tube-like structures that yield very -attractive cabochons and cabinet specimens. Texas fossil palm wood is -highly regarded by cutters from all parts of the country, and this -material is thought by many lapidaries to be some of the finest -gem-quality fossil wood in the United States. - -Gravel pits and river gravels in Live Oak County have produced very fine -agatized wood. Although the gem material does not seem to be as abundant -in this area as it is in counties to the northeast, the vivid colors and -excellent preservation of the fossil wood in Live Oak County have -attracted collectors from all over the State. The fossil wood usually -occurs as large rounded cobbles in the streams. Much of this material is -quite translucent when cut and contains various shades of brown, orange, -and red. - -The gravels of the Rio Grande have produced some fossil wood in addition -to the excellent agate that is also found there. Most of the fossil wood -found in these gravels is very well preserved, but the colors are -commonly dull shades of brown. Occasional fine red and yellow specimens -have been recovered from the Rio Grande gravels, but these are rare. - -Good agatized wood has been found in and near Palo Duro Canyon, -Armstrong County, about 50 miles southeast of Amarillo. Large trunk -sections are not uncommon, but most of the material of cutting quality -is obtained from small fragments. The Palo Duro Canyon fossil wood -greatly resembles the famous Arizona Petrified Forest wood but is not -nearly as plentiful. The Palo Duro wood contains yellow, brown, red, and -bluish colors most commonly. Some of the wood-producing area is within -Palo Duro Canyon State Park which is, of course, closed to collecting. -The surrounding area has been worked diligently by local collectors, but -new pieces of wood are exposed after heavy rains. - -Webb and Duval counties have also produced some good fossil wood -specimens. - - - - - Gadolinite - - - _Composition_: Be₂FeY₂Si₂O₁₀. (Various other rare-earth elements may - substitute into this mineral structure.) _Crystal system_: monoclinic. - _Hardness_: 6.5 to 7.0. _Specific gravity_: about 4.2. _Luster_: - vitreous to greasy. _Color_: black; in thin splinters dark bottle - green. _Streak_: white to greenish. _Cleavage_: none. _Fracture_: - conchoidal to splintery. _Tenacity_: brittle. _Diaphaneity_: opaque to - subtransparent in thin pieces. _Refractive index_: variable, about - 1.77 to 1.82. - -Gadolinite as a cut gem is not seen outside of large collections; -however, it can be faceted into black opaque stones of little beauty but -of great interest to collectors. The best known locality of this mineral -in the United States is Baringer Hill, Llano County, Texas. -Unfortunately, this locality was completely flooded by the completion of -Buchanan Dam in 1938. Masses and rough crystals of gadolinite weighing -over 100 pounds were mined from this locality. The gadolinite occurred -in a large, very coarse-grained pegmatite dike associated with quartz, -microcline, and fluorite, as well as allanite, fergusonite, nivenite, -cyrtolite, thorogummite, and various other rare minerals. Some of the -minerals in the dike occurred in very large masses. One quartz mass over -40 feet in diameter was noted, and microcline masses up to 30 feet in -diameter were not uncommon. Much of the gadolinite was used by -industrial firms as a source of thorium compounds, although some -specimen and gem material found its way into museums and private -collections. Because the locality was worked mostly from 1910 to about -1925 and because since 1938 the waters of Lake Buchanan have completely -flooded the entire area, material from this locality is now exceedingly -difficult to obtain. The collection of the Smithsonian Institution, -Washington, D.C., contains a cut and polished gem of Baringer Hill -gadolinite that weighs 8.6 carats. This mineral is radioactive because -of the presence of uranium, thorium, and other rare radioactive -elements. - - - - - Garnet - - -The garnet group of minerals is variable in composition. Listed below -are the pure members of this group, but garnets found in nature are -usually a mixture of two or more of these end members. - - Aluminum garnet— - Grossularite (calcium-aluminum garnet), Ca₃Al₂(SiO₄)₃ - Pyrope (magnesium-aluminum garnet), Mg₃Al₂(SiO₄)₃ - Almandite (iron-aluminum garnet), Fe₃Al₂(SiO₄)₃ - Spessartite (manganese-aluminum garnet), Mn₃Al₂(SiO₄)₃ - Iron garnet— - Andradite (calcium-iron garnet), Ca₃Fe₂(SiO₄)₃; may contain - magnesium, titanium, and yttrium - Chromium garnet— - Uvarovite (calcium-chromium garnet), Ca₃Cr₂(SiO₄)₃ - -Since almandite is the only variety of garnet known to occur commonly in -gem quality in Texas, the following properties are for almandite except -where noted. - - _Crystal system_: isometric (all varieties). _Hardness_: about 7.5. - _Specific gravity_: 4.25. _Luster_: vitreous to resinous. _Color_: - red, deep red, and brownish red (other varieties also yellow, white, - orange, pink, black, and green). _Streak_: white. _Cleavage_: none. - _Fracture_: subconchoidal to uneven. _Tenacity_: brittle to tough. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: - about 1.83. - -Good crystals of gem-quality almandite garnet have been found in Llano, -Blanco, Burnet, and Gillespie counties. In southeast Llano County, -northwest Blanco County, and northeast Gillespie County, the stones -mostly occur in stream gravels where they have collected after being -weathered out of compact mica schists. Owing to the fact that most of -the garnets have not been transported very far from their source, the -stones commonly show good crystal form (Pl. II, A). All of the garnets -from one locality commonly do not have exactly the same crystal form. -The garnets are mostly widely scattered in the stream gravels but can be -found concentrated behind rocks and on small gravel bars. - -Many of the crystals are less than one-eighth inch in diameter; however, -good crystals one-fourth to one-half inch in diameter are common. Most -of the stones are too fractured or have too many inclusions to yield -gems, but many transparent stones have been found. The transparent -crystals usually yield flawless deep red faceted stones of 2 carats or -less. Some of the stones that contain too many inclusions to facet are -cut as cabochons and are then often known as carbuncle. - -Small garnet fragments have been found in streams and in gneisses and -pegmatites near Castell, Llano County, but they are not commonly of gem -quality. - -Occasional small gem-quality garnets have been found in pegmatites and -contact metamorphic zones in Burnet County. Garnets have also been found -in several other counties, notably Mason, El Paso, Hudspeth, and -Culberson, but no stones of facet quality have been reported. - - - - - Jet - - - _Composition_: a variety of brown coal or lignite. _Structure_: woody. - _Hardness_: 3 to 4. _Specific gravity_: about 1.30 to 1.35. _Luster_: - dull. _Color_: black, brownish black. _Streak_: brown to brownish - black. _Cleavage_: none. _Fracture_: uneven to smooth. _Tenacity_: - tough to slightly brittle. _Diaphaneity_: opaque. Burns with a sooty - yellowish flame. - -Jet is a type of fossil wood in which there has been sufficient chemical -change to make the wood relatively hard and black without destroying the -woody structure. The best specimens of jet polish into lustrous black -cabochons. - -Jet occurs in Presidio County as compressed and flattened trunks of -trees in a thin layer of coal and lignite in Cretaceous strata 100 to -200 feet stratigraphically below the San Carlos beds. - -Specimens of “jet” have been found in some of the lignitic Tertiary -strata of the Texas Gulf Coastal Plain; however, this material is mostly -soft, brownish, and not of gem quality. - - - - - Labradorite - - - _Composition_: NaAlSi₃O₈, 50% to 30%; CaAl₂Si₂O₈, 50% to 70%. _Crystal - system_: triclinic. _Hardness_: 6.0 to 6.5. _Specific gravity_: about - 2.60. _Luster_: vitreous to sometimes pearly. _Color_: straw yellow, - white, greenish, gray, reddish, bluish, and green. Sometimes shows a - play of colors on particular cleavage surfaces. _Streak_: uncolored. - _Cleavage_: three directions. _Fracture_: uneven to conchoidal. - _Tenacity_: brittle. _Diaphaneity_: transparent to translucent. - _Refractive index_: about 1.56. _Dispersion_: low. - -Very fine facet-quality labradorite has been found about 20 miles south -of Alpine, Brewster County. The labradorite occurs loose in the soil as -slightly weathered or frosted cleavage fragments, commonly showing one -or more crystal faces (Pl. II, B). The pale-yellow or straw-yellow color -of these fragments, as well as their lack of internal imperfections, -makes these stones excellent gem material. Individual pieces that exceed -three-fourths inch in their longest dimensions are rare. Cut stones of -more than 5 or 6 carats from this locality are scarce. The source of -this material is uncertain, but it is probably weathering out of an -underlying igneous rock. - - - - - Microcline - - - _Composition_: KAlSi₃O₈. _Crystal system_: triclinic. _Hardness_: 6.0 - to 6.5. _Specific gravity_: 2.54 to 2.57. _Luster_: vitreous to - pearly. _Color_: white, pale yellow, red, blue green, bluish. - _Streak_: white. _Cleavage_: four directions, usually three of these - distinct. _Fracture_: uneven. _Tenacity_: brittle _Diaphaneity_: - transparent to translucent. _Refractive index_: about 1.52 to 1.53. - -Very fine crystals of blue microcline have been found east of Packsaddle -Mountain and near Kingsland in Llano County. Crystals exceeding 1 foot -in length have been found, although most are only a few inches long. The -color of the microcline is mostly pale blue, but some crystals are -darker. Microcline crystals associated with milky or vein quartz, smoky -quartz, some biotite, and rarely cassiterite occur in pegmatite dikes -which vary in size from a few inches to several feet in thickness. The -color of this microcline is pale in comparison to microcline from some -other localities in the United States, but the Texas blue microcline -does yield pleasing cabochons. Perfect crystals of this material are -prized by collectors. Blue or greenish microcline is often called -amazonite or amazon stone. - -Bluish microcline associated with quartz and topaz has also been -reported near Katemcy, Mason County. - -Red microcline is common in several central Texas counties and is a -primary constituent of many of the igneous rocks in those counties. -Large crystals of perthitic red microcline occur in pegmatite dikes of -Mason, Llano, Burnet, and Gillespie counties. Any feldspar quarry or -other pegmatite mining operation in any of these counties is likely to -contain large red microcline crystals and fragments. Unfortunately, the -good crystals that may have been present are often shattered by blasting -during quarrying operations. - -Feldspar quarries in northeastern Gillespie County have yielded some -good red cabochon material as well as good crystals. Here the microcline -occurs with milky and smoky vein quartz, smoky quartz crystals, clear -quartz crystals, greenish muscovite, and biotite. Many of the older -quarries in Gillespie County have not been active for some time, and the -dumps and quarry walls have been diligently searched by collectors. - - [Illustration: Fig. 16. Crystal faces on microcline specimen shown in - Plate III, A.] - -Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet -counties have produced some good red microcline specimens and cutting -material (Pl. III, A, and fig. 16). Many of these crystals are more -pinkish than those in Gillespie County, but this is commonly due to the -fact that the crystal faces of the Lake Buchanan area crystals are -somewhat more weathered than the fresh Gillespie County crystals. - -Numerous other local areas in the counties mentioned, as well as some -localities in Hudspeth and Culberson counties, have also produced small -amounts of red and pink microcline of gem quality. - - - - - Obsidian - - - _Composition_: volcanic glass. _Structure_: amorphous. _Hardness_: 5.0 - to 5.5. _Specific gravity_: 2.3 to 2.5. _Luster_: vitreous. _Color_: - black, dark gray, reddish, brown, bluish, and greenish. _Streak_: - white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: translucent to nearly opaque. _Refractive index_: - variable, about 1.45 to 1.53. - -Gem-quality black and dark-gray obsidian has been found in Presidio -County associated with extrusive igneous rocks. The obsidian in this -area is too opaque to serve as attractive faceted stones but is found in -pieces of sufficient size and quality to yield nice cabochons. Some of -the small weathered pieces of this material resemble tektite in outward -appearance; in fact, the “valverdites” mistaken originally for tektites -are pebbles of weathered obsidian in terrace gravel of Val Verde County. -Obsidian takes a high polish but is very sensitive to heat. Stones that -are slightly overheated during grinding or sanding will quickly shatter. - -Obsidian of gem quality has been reported also in Brewster County. - - - - - Opal - - - _Composition_: SiO₂·nH₂O. _Structure_: amorphous. _Hardness_: 5.5 to - 6.5. _Specific gravity_: 1.9 to 2.3. _Luster_: subvitreous to pearly. - _Color_: white, bluish, pink, brown, yellow, and gray. _Streak_: - white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: transparent to nearly opaque. _Refractive index_: 1.43. - -Opal other than as fossil or opalized wood (pp. 20-21) occurs at the -following several localities in Texas. - -Approximately 16 miles south of Alpine, Brewster County, precious opal -occurs in very small seams and as cavity fillings in very hard -pinkish-brown rhyolite. This opal is milky or bluish and commonly -exhibits small flashes of blue, green, red, and orange fire. Individual -pieces of this opal are mostly quite small, rarely over one-fourth inch -in diameter, and very difficult to remove from the tough rhyolite -matrix. Local lapidaries have cut interesting cabochons from this -material in which several small patches of opal that are close together -in the matrix are included in the same cabochon. - -Small finds of opal associated with rhyolites and basalts have come from -other localities in west Texas, but the opal mostly does not display -enough play of colors to warrant its use as gem material. - -Near Freer, Duval County, some very attractive common opal has been -found. The opal is colored various shades of pink, blue, and yellow and -in certain local areas occurs as fragments that are cemented together by -clear chalcedony. Various colors are commonly found in the same piece, -and such material yields handsome cabochons. Although the area has never -been worked commercially, it has been hunted by collectors and cutters -for several years. - - - - - Pearl - - -Pearls are the result of the secretion of calcium carbonate by various -shellfish around sand grains, parasitic organisms, shell fragments, or -other foreign objects that have in some way entered the body cavity of -the shellfish. Since the shellfish is unable to expel these irritating -particles or organisms, it deposits successive layers of calcium -carbonate around the foreign substance to make it smoother and less -irritating. Although pearls are principally calcium carbonate, they also -contain small amounts of an organic substance, called conchiolin, and -water. Pearls are found in shellfish that live in either fresh or salt -water. Few pearls are spherical in shape; most are rounded but somewhat -irregular and are known as baroque pearls. Good quality pearls are the -only gemstone commonly sold by the grain, a unit of weight equal to 0.25 -carat or 0.05 gram. The pearl grain is not the same unit of weight as -the Troy grain. - -In Texas, pearls have been found in fresh-water clams in most of the -major rivers and streams, notably in the Brazos, Concho, Colorado, -Guadalupe, Llano, Nueces, Sabine, Rio Grande, and Trinity Rivers. -Several Texas lakes have also yielded pearls, notably Caddo Lake and -other lakes in north-central and northeast Texas. - -Small pearls are frequently found along the Texas Gulf Coast in edible -oysters and other common shellfish. Fossil pearls have also been found -but because of their darkened appearance are of value only as -curiosities. - -The pearls thus far found in Texas have been of relatively poor quality -and show little or no iridescence. These pearls have little value except -as curiosities, although one writer has stated that the discovery of -pearls in the Nueces River led to the original Spanish settlement of the -State (Baker, 1935, p. 569). - - - - - Quartz - - - _Composition_: SiO₂. _Crystal system_: hexagonal. _Hardness_: 7. - _Specific gravity_: 2.65 to 2.66 in crystals. _Luster_: vitreous, also - waxy, greasy, and dull. _Color_: most often colorless, brown, yellow, - violet; sometimes green, red, blue, and black; cryptocrystalline - varieties often variously colored by impurities. _Streak_: white. - _Cleavage_: indistinct. _Fracture_: conchoidal to splintery. - _Tenacity_: brittle to tough. _Diaphaneity_: transparent to opaque. - _Refractive index_: 1.544 to 1.553. - -The quartz family gemstones can be divided into two groups for purposes -of description. The first group is the crystalline varieties, or those -quartz varieties that commonly occur in distinct crystals. The second -group is the cryptocrystalline varieties, or those quartz varieties that -occur as irregular masses that are composed of many microscopic -crystals. The crystalline varieties are usually much more transparent -and are most often seen as faceted stones. The cryptocrystalline -varieties vary from subtransparent to opaque and are almost always cut -as cabochons. - - - - - CRYSTALLINE VARIETIES - - -_Amethyst_ (violet to purple-colored quartz).—A northeastern Gillespie -County locality known as Amethyst Hill has produced quite a number of -fine light to medium violet amethyst crystals which occur in quartz -veins and geodes associated with serpentine and talc. Many crystals have -been found loose in the soil. - -The amethyst tends to be very irregularly colored in zones parallel to -the crystal faces. In many, the base of the crystal is colorless or -white and only the termination is violet. Crystals up to 3 inches long -have been found at this locality, but the average size is much less. - -The surface at this locality is almost entirely depleted of amethyst, -with only an occasional small crystal or fragment to be seen. However, -small excavations are still sometimes productive. - -Good groups of pale amethyst crystals have been found in quartz veins -near the old town site of Oxford, Llano County. The occurrence seems to -be much the same as the Amethyst Hill locality. Little exploration for -gemstones has been done in this area, and future discoveries seem -likely. - -Chalcedony geodes lined with amethyst crystals have been found in -Brewster, Presidio, Culberson, and Hudspeth counties, but the -occurrences are scattered. The crystals are seldom large enough to yield -gems of more than 3 carats and are mostly very light colored. - -A few pieces of gem-quality amethyst have been found in Burnet County. - -_Citrine_ (yellow quartz).—Very little gem-quality citrine has been -reported in Texas. Some small citrine crystals have been found at -Amethyst Hill in northeastern Gillespie County, but few are of -sufficient size or color to yield good gems. - -The writer has seen one citrine crystal that was found in the gravels of -a small stream in eastern Llano County near Buchanan Dam. The crystal -weighs about 1 ounce and is perfectly clear, light golden yellow, and -flawless. However, a further search of the stream gravels failed to -produce any other citrines. - -_Rock crystal_ (colorless quartz).—Numerous localities in Texas produce -this colorless variety of quartz, which is the most common variety of -facet quality quartz and consequently is of little value. - -Rock crystal occurs at many localities in Burnet, Llano, and Mason -counties. The crystals mostly occur in pegmatite dikes or in stream -gravels where they have been weathered out of their parent rock. Some -fine colorless quartz crystals have been found near Voca, Mason County, -in weathered pegmatite dikes and also loose in the sands of nearby -streams. Crystals from this locality are often stained with reddish iron -oxide on their outer surfaces. Some of the rock crystal found near -Katemcy, Mason County, shows asterism when cut with the proper -orientation. Fine clear colorless crystals up to 8 inches long have been -found in the pegmatite dikes near Lake Buchanan in both Llano and Burnet -counties. Several localities near Enchanted Rock in Llano County have -also produced some good colorless crystals. - -Feldspar quarries in large pegmatites in northeastern Gillespie County -have yielded attractive quartz crystals, some of which contain smoky -phantom crystals and tourmaline inclusions. - -Some pieces of rock crystal enclosing green, needle-like actinolite -crystals have been found near the Llano-Gillespie-Blanco County corner. -This material is not suitable for faceted gems but does lend itself to -interesting and attractive cabochons. - -Colorless quartz crystals commonly are found lining small chalcedony -geodes in Brewster, Presidio, Culberson, Hudspeth, Reeves, and Jeff -Davis counties. These crystals are most commonly less than 1 inch long -but are mostly very clear. - -Rock crystal has been found in crevices of petrified wood in many east -and southeast Texas counties, although the crystals are mostly quite -small. - -Many lesser occurrences of rock crystal, too numerous to mention, are -located within the State. - -_Rose quartz_ (pink quartz).—Rose quartz occurs at various localities in -Burnet, Llano, Mason, and Gillespie counties, but the amount of material -is mostly small and the greater part unsuitable for gem purposes. Some -good pink rose quartz occurs near Town Mountain, Llano County, but this -material does not have flawless areas large enough to yield faceted -stones of more than a few carats. Rose quartz is always slightly milky, -or cloudy, and does not cut into brilliant faceted stones. The Town -Mountain rose quartz has been cut into attractive cabochons. - -_Smoky quartz_ (brown, yellow-brown, and golden-brown quartz).—Several -Texas localities have produced fine smoky quartz. Baringer Hill, a noted -rare-earth minerals pegmatite locality in Llano County, contained some -smoky quartz crystals that were estimated to weigh over 1,000 pounds, -and the locality produced many smaller crystals that were of gem -quality. Baringer Hill was flooded by the completion of Buchanan Dam in -1938 and is presently under the waters of Lake Buchanan. A few fine -golden-brown gem-quality crystals have been found along the lake shore -and in small pegmatites nearby (Pl. III, B.). - -Feldspar quarries in northeastern Gillespie County have produced smoky -quartz crystals that exceed 1 foot in length, but these crystals are -mostly flawed, possibly as a result of blasting, and mostly contain only -small clear areas. - -Good color smoky quartz crystals are found with topaz in the pegmatites -and stream beds in Mason County, near Streeter, Grit, and Katemcy. These -crystals tend to be lighter colored than those near Lake Buchanan, but -they commonly contain large flawless areas. - - - - - CRYPTOCRYSTALLINE VARIETIES - - -_Chalcedony._—When free from impurities of various oxides and other -compounds, chalcedony has little to render it pleasing as a gemstone. It -is mostly gray, white, brown, or bluish and commonly has a waxy luster. -Some of the chalcedony found along the Rio Grande Valley and in west -Texas will take dyes, and local lapidaries have had some success in -dyeing this material various shades of blue, green, yellow, and red. -When the chalcedony is naturally colored and variegated, usually in -bands, mossy figures, or dendritic forms, it is called agate. - -_Agate_ (variegated chalcedony).—The wide variety of markings and colors -available together with the ease of cutting make agate a favorite of -many lapidaries. Fine agate has been found at numerous localities in -west and south Texas. Fine plume agate, famous throughout the United -States, is found south of Alpine. Plume agate is characterized by -dendritic or tree-like inclusions and is mostly cut into very handsome -cabochons. The agate from south of Alpine commonly contains black, red, -yellow, or brown plumes within the same piece. The variety of colors and -lack of porosity of this agate make it highly desired among lapidaries. -The agate occurs loose on the surface of the ground and in the soil in -small nodules that have a very rough, brownish surface. These nodules -are mostly less than 3 inches in diameter, although specimens of gem -quality have been found that exceed 200 pounds. - -Some very fine agate has been found in the vicinity of Needle Peak, -Presidio County. This material is mostly green moss agate in clear -chalcedony and commonly contains small yellow “sun-burst” figures. The -contrasting yellow and green design makes very beautiful cabochons. - -Fine agate has been found south of Marfa, Presidio County. This agate is -mostly clear chalcedony with black, yellow, or variously colored plumes, -moss, or “bouquet-like” figures. - -Numerous other localities in Presidio and Brewster counties have -produced good agate. - -Various amounts of agate, jasper, and chalcedony occur in the gravels of -the Rio Grande in varying quantities from Big Bend National Park -downstream to Brownsville. This agate is found both in the present river -gravels and in the older river gravels that now are located on nearby -hills and slopes up to several miles north or south of the present Rio -Grande. The greatest concentration of agate and related gem materials -seems to be in the area between Laredo and Rio Grande City. Vast -quantities of excellent gem material have been removed from this area -for many years (Pl. IV). The agate occurs as rounded, stream-worn -cobbles and commonly has a thin white coating that makes it difficult to -distinguish from the abundant chert and other rocks. The agate occurs in -cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem -quality that exceed twice this size are known. The agate varies greatly -in design and color. Plume, moss, banded, and sagenitic agate occur in -these gravels in a wide variety of colors. The jasper in the Rio Grande -gravels is yellow, red, green, or various shades of these and is -commonly suspended as angular fragments in clear chalcedony. - -Good agate has also been found near Balmorhea in Reeves and Jeff Davis -counties and in smaller amounts at numerous other west and south Texas -localities. - -_Agatized wood_ (_see_ Fossil wood, pp. 20-21). - -_Carnelian_ (translucent reddish chalcedony).—This variety of chalcedony -in small quantities has been reported from near Van Horn, Hudspeth -County. Small pieces of carnelian have been found in the gravels of the -Rio Grande, but finds have been few and scattered. - -_Jasper_ (impure opaque or subtranslucent quartz).—Good green, yellow, -red, and brown jasper has been found in the gravels of the Rio Grande at -all of the localities that produce agate. The colors are quite vivid, -and the material takes a fine polish. Some pieces of orbicular jasper -(jasper with circular or eye-like markings) have been found in this -material. These gravels commonly contain jasper as fragments that are -suspended in clear chalcedony; this is called brecciated jasper and -yields very handsome cabochons. - -Many of the west Texas agate localities also produce jasper in quantity. -Good jasper has been reported from north of Brackettville, Kinney -County. Jasper is a minor constituent of the stream gravels in many -parts of the State. - - - - - Sanidine - - - _Composition_: KAlSi₃O₈; commonly contains some sodium. _Crystal - system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58. - _Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow, - and gray. _Streak_: uncolored. _Cleavage_: three directions. - _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: - transparent to subtranslucent. _Refractive index_: 1.52 to 1.53. - -Some feldspars, including sanidine, show a nice blue sheen in reflected -light parallel to certain crystallographic directions. Stones having -this property are called moonstone. A clear yellowish sanidine showing -an attractive blue sheen has been found in Brewster, Jeff Davis, and -Presidio counties. The individual pieces are small, the average size -being about one-eighth inch. The sanidine is found loose in the soil at -some localities where it has weathered out of rhyolite, and specimens of -the sanidine in the parent rock are not difficult to obtain. Very small -cabochons can be cut from this material, but few lapidaries have done so -because inexpensive larger pieces of moonstone can be obtained easily -from foreign sources. However, the west Texas sanidine does show a blue -sheen when cut and polished. - - - - - Spinel - - - _Composition_: MgAl₂O₄ (magnesium may be replaced in part by ferrous - iron or manganese and the aluminum by ferric iron and chromium). - _Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to - 4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red, - blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_: - one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: transparent to opaque. _Refractive index_: variable, - approximately 1.72 to 2.00. - -In many areas of the world, fine quality, beautifully colored, -transparent spinels are found and used as gems. The only gem-quality -spinel reported thus far in Texas is black and opaque. Near Eagle Flat -in Hudspeth County, black spinel crystals have been found associated -with augite and natural glass; these minerals are weathering out of an -intrusive igneous rock. The spinel crystals have an octahedral form -which is common for this mineral (fig. 17). Most of the spinels are free -of flaws, but because of their black color they have little value as -gems. The crystals are found loose in the sand of streams near the -outcrops of the igneous rock or embedded in the rock. They seldom exceed -half an inch in diameter. These stones are primarily sought by -collectors. - - [Illustration: Fig. 17. Common crystal form of spinel.] - - - - - Tektite (Bediasite) - - - _Composition_: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4% - FeO, also MgO, Na₂O, K₂O, and traces of other elements. _Crystal - structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to - 2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_: - dark brown, greenish brown, appears black in thick sections. _Streak_: - uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: - brittle. _Diaphaneity_: transparent to subtransparent. _Refractive - index_: 1.488 to 1.512. - -The average bediasite size is about 1 inch in diameter, although -specimens approximately 3 inches in diameter are known. The uncut -tektites are very interesting, showing a variety of shapes and surface -features (Pl. V, A) and many exhibit contorted flow structure. The -surface of many tektites is grooved or furrowed, while on others it is -smooth or frosted. The Texas tektites are known as “bediasites,” after -place names in Grimes County traceable to the Bedias Indians who -formerly lived there. - -Dark brown and greenish-brown tektites have been found in Texas in -gravels at scattered localities in Walker, Grimes, Brazos, Burleson, -Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas -the only other authenticated tektite localities in the United States at -the present time are in Dodge and Irwin counties, Georgia. A fragment of -a similar tektite has recently been reported from near Martha’s -Vineyard, Massachusetts. The tektites reported from Oklahoma are now -known to be pebbles of obsidian. - -Although tektites have little value or beauty as gemstones, they have -been cut by lapidaries as both faceted and cabochon stones. Tektites -take a high polish but are mostly so dark in color that they appear -black. - -The origin of tektites is of great scientific interest and is currently -the subject of much debate. Some scientists believe that tektites are of -meteoritic origin, while others believe that tektites were formed by -various terrestrial processes. Since no one has actually observed a -tektite to fall or form, and many of the theories of origin are -difficult to prove without direct observation, the origin of tektites is -likely to remain in controversy for some time. - - - - - Topaz - - - _Composition_: Al₂(F, OH)₂SiO₄. _Crystal system_: orthorhombic. - _Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous. - _Color_: pale blue, sky blue, greenish, white, wine yellow, straw - yellow, grayish, pink, reddish, and orange. _Streak_: uncolored. - _Cleavage_: one direction, basal, highly perfect. _Fracture_: - conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent - to subtranslucent. _Refractive index_: about 1.60 to 1.63. - _Dispersion_: moderate. - -Various yellow and smoky colored quartz gems are offered for sale as -“Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.” -These names are entirely misleading and should be dropped from usage. - -Fine gem-quality white, pale-blue, and sky-blue topaz has been found -near Streeter, Grit, and Katemcy, Mason County. This Texas gem material -compares favorably in color, size, and clarity with topaz found anywhere -in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18) -occasionally are found in pegmatite dikes associated with quartz, black -tourmaline, cassiterite, and pink microcline. Many of the gem-bearing -pegmatites have been eroded away, leaving the topaz concentrated in the -stream beds. The stones mostly occur as frosted, stream-worn pebbles -(Pl. VI, A) in the numerous small creeks in the area. The topaz is -heavier than the quartz and microcline that compose the stream gravel -and is commonly found immediately on top of the granite bed-rock in the -bottom of the stream bed. The stones tend to lodge behind boulders or -small dikes cutting across the stream. - - [Illustration: Fig. 18. Crystal faces on topaz crystal shown in Plate - V, B. This crystal habit is typical of the topaz from Mason County.] - -The white or colorless stones are by far the most common, outnumbering -the bluish stones about ten to one. The color of the blue stones tends -to be irregularly distributed in zones parallel to the crystal faces. -Topaz that is colored in this manner should be cut with the best blue -color near the bottom or culet of the gem (fig. 19). If done correctly, -this will give the entire gemstone the desirable blue color. - -[Illustration: Fig. 19. Cross section showing the proper orientation of - dark-color zone in a gem cut from an irregularly colored stone.] - - COLORLESS - BLUE - -The colorless stones can be turned pale yellow, yellowish brown, or -straw yellow by exposure to X-ray radiation, and some of the bluish -stones will fluoresce faintly yellowish under ultra-violet light. - -The largest gem-quality topaz crystal yet found in North America has -come from Mason County. It is a pale-blue crystal weighing 1,296 grams, -now in the collection of the U.S. National Museum. Several other large -pieces, some weighing over a pound, have been found. One large crystal, -exact weight unknown, was found near Katemcy. Several gem cutters have -estimated that this stone could easily yield a single, flawless -pale-blue gem of about 500 carats. Many large gems have been cut from -topaz found in this area, including at least one stone of over 300 -carats. - -One obstacle in the cutting of topaz is its perfect basal cleavage. The -gemstone should be oriented so that no facet of the stone will be -parallel to or within less than about 5 degrees of the cleavage -direction, or the facet may be very difficult or impossible to polish. - -It is difficult to estimate the productivity of this area since its -discovery in the early 1900’s. Few systematic attempts have been made to -exploit the deposits, and a great amount of the topaz thus far recovered -has been found by private collectors. The Mason County topaz deposits -are still very productive, and additional exploration may uncover even -more gem-producing areas. - -Topaz has also been found in stream gravels or pegmatites in Burnet, -Llano, Gillespie, and El Paso counties but very rarely in gem quality. - - - - - Tourmaline - - - _Composition_: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron, - magnesium, calcium, or fluorine. _Crystal system_: hexagonal. - _Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_: - vitreous to resinous. _Color_: black, brownish black, brown, blue, - green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_: - two directions, very imperfect. _Fracture_: subconchoidal to uneven. - _Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive - index_: about 1.62 to 1.64. - -Black tourmaline is schorl; brown tourmaline, dravite. - -Good crystals of black and dark brown tourmaline occur at Town Mountain -near Llano, Llano County. The tourmaline crystals average about 1 inch -in length, do not commonly exceed 2 inches, and are associated with -white vein quartz. The quartz completely encloses the tourmaline, but -the crystals can be broken free or the quartz can be trimmed away with -the use of a diamond saw. The latter procedure is recommended whenever -possible, for it is very easy to shatter the tourmaline crystals while -trying to remove them from the quartz by other means. Many of the -crystals are completely unsuitable for cutting, being too brittle or too -badly cracked and flawed. However, some small crystals have been found -that are of sufficient quality and size to yield flawless stones of a -few carats. Few of these stones have been cut since the tourmaline is so -dark that it appears opaque, and few persons find a gem of this nature -attractive. - -Good black and dark brown crystals of tourmaline associated with -andalusite and graphite occur in the Packsaddle schist (Precambrian) -near Sunrise Beach, Llano County (Pl. VI, B, and fig. 20). Although -generally smaller in diameter than the crystals found at Town Mountain, -they commonly exceed 3 inches in length, although the average size is a -little over 1 inch. Many of these crystals are suitable for cutting into -opaque or nearly opaque stones of about 5 or 6 carats. - -Black tourmaline has also been found in Hudspeth and Culberson counties -but not of sufficient quality to be used as a gemstone. - -[Illustration: Fig. 20. Common crystal form of Llano County tourmaline.] - - - - - Turquoise - - - _Composition_: hydrous phosphate of aluminum and copper. _Crystal - system_: triclinic. _Hardness_: 5 to 6. _Specific gravity_: variable, - 2.6 to about 2.8. _Luster_: dull, sometimes waxy. _Color_: sky blue to - greenish blue. _Streak_: white to greenish. _Cleavage_: none in - massive material, two directions in crystals. _Fracture_: conchoidal - to subconchoidal. _Tenacity_: brittle. _Diaphaneity_: subtranslucent - to opaque. _Refractive index_: 1.61 to 1.65. - -Turquoise of good sky-blue to greenish-blue color has been found a few -miles southwest of Van Horn, Culberson County. Several shallow pits were -dug at this locality about 1910; however, the amount of turquoise -produced was small. The main occurrence of the turquoise was in seams -about 1 millimeter thick along joints in the fine-grained rocks of this -area. Persons who have visited Culberson County more recently report -that even minute traces of the turquoise are now difficult to find at -the old prospect pits. However, further prospecting in the area might -yield some additional localities. - -Small amounts of turquoise have been reported near El Paso, El Paso -County, and also in volcanic rocks near the Jeff Davis-Brewster County -line, north of Alpine. - -A small amount of turquoise has been mined from several localities a few -miles northwest of Sierra Blanca in the Sierra Blanca Mountains of -Hudspeth County. - - - - - GLOSSARY - - - Amorphous—without definite molecular structure; not crystalline. - Baroque stone—an irregularly shaped, polished stone; usually applied - to tumbled stones. - Baroque pearl—an irregularly shaped pearl. - Brilliancy—reflecting much light; having brightness. - Brilliant cut—a mode of arrangement of facets commonly used on round - or oval stones. The standard American brilliant cut has 57 or - 58 facets. Most diamonds of 5 or less carats are cut in this - manner. - Cabochon—a stone cut with a flat or convex upper surface; sometimes - faceted in part. Opal, star sapphire, and agate are stones - that are frequently cut in this style (fig. 2). - Cambrian—a division of geologic time, estimated to be the time from - 550 to 440 million years ago; the oldest time division of the - Paleozoic era. - Carat—a unit of weight equal to ⅕ of a gram or 0.2 gram. One ounce - avoirdupois is equal to 141.75 carats. - Cleavage—the tendency of certain minerals to split in particular - directions yielding relatively smooth plane surfaces. - Conchiolin—an organic albuminoid substance found in pearls. - Conchoidal—a type of fracture having curved concavities or the - approximate shape of one-half of a bivalve shell. Glass has - excellent conchoidal fracture. - Cretaceous—a division of geologic time, estimated to be the time from - 135 to 60 million years ago; youngest division of the Mesozoic - era. - Crown—that portion of a faceted gem above the girdle; the upper - portion of a facet-cut gem (fig. 6). - Cryptocrystalline—composed of very fine or microscopic crystals. - Crystal—the regular polyhedral form, bounded by plane surfaces, that - is assumed by a mineral under suitable conditions. Crystals - have definite external symmetry and internal molecular order. - Crystalline—possessing definite internal molecular order; not - amorphous. - Cubic—in the general shape of a cube. The isometric crystal system is - often called the cubic system. - Culet—the very bottom portion of a faceted gem; the point or line - formed by the intersection of the lowest pavilion facets (fig. - 6). - Dendritic—branching or tree-like in form. - Diaphaneity—relative transparency. The diaphaneity of a mineral is - described as transparent, translucent, opaque, etc. - Dike—a tabular rock body, usually igneous in origin, which cuts across - the surrounding rock strata. - Dispersion—a measure of the ability of gemstones to separate complex - or white light into its component colors; often illustrated - with a prism. Gemstones that are capable of separating colors - of light widely are said to have high dispersion; gemstones - not so capable of separating white light into colors are said - to have low dispersion. - Dopping—the act of cementing a gemstone, either rough or partly - finished, to a dop-stick. - Dop-stick—the wooden stick or cylindrical piece of metal to which a - gemstone is cemented to facilitate handling during cutting and - polishing. - Dop-wax—the agent or cement used to secure a gemstone to a dop-stick. - Emerald cut—a rectangular or square faceted stone with beveled corners - whose surfaces are covered with several series of rectangular - facets. - Eocene—a division of geologic time, estimated to be the time from 50 - to 40 million years ago; one of the older divisions of the - Cenozoic era. - Extrusive rock—igneous rock that has been extruded or forced out onto - the earth’s surface. - Facet—a single plane polished surface on a faceted gem. - Facet head—a device used in the cutting and polishing of faceted gems; - used to control the placement of facets and their relative - angles (fig. 7). - Facet table—the equipment used in the cutting and polishing of faceted - gems and the table on which most of the equipment is mounted - (fig. 7). - Feldspar—a group of closely related silicate minerals including - orthoclase, microcline, sanidine, plagioclase, labradorite, - and others. - Fire—the reflections of variously colored light from a precious opal; - also the different colors of light reflected from a faceted - gem owing to the dispersion of the mineral. - Fracture—the texture of a freshly broken surface other than a cleavage - surface, described as conchoidal, even, splintery, etc. - Gem—a cut and polished gemstone. - Gemology—the science dealing with the study of gemstones. - Gemstone—a mineral suitable for cutting into a gem; the term gemstones - is frequently used collectively to include both cut and - polished stones and rough stones. - Geode—a rounded or spherical rock cavity; commonly lined with - crystals. - Girdle—the portion of a faceted gem separating the crown from the - pavilion; the girdle may or may not be polished and usually - contains about 2 percent of the total depth of the gem (fig. - 6). - Gneiss—a coarse-grained metamorphic rock having segregations of - granular and platy minerals that give it a more or less banded - appearance without well-developed schistosity. - Grain (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat; - not the same as the Troy grain. - Granite—a granular igneous rock composed mostly of quartz, feldspar, - and commonly mica and/or hornblende. - Hexagonal—having six angles and six sides; a crystal system in which - the crystal faces are referred to four intersecting axes; - three of these axes are equal, lie in the same plane, and - intersect at angles of 60 degrees; the fourth axis is - perpendicular to the other three. - Igneous rock—rock formed by solidification from a hot melt. - Index of refraction—a measure of the relative ability of a gemstone to - “bend” incident light rays; sine of the angle of incidence of - a light ray divided by the sine of the angle of refraction. - Intrusive rock—rock that has been pushed (usually in a molten state) - among pre-existing rock strata, commonly along faults or - fissures. Intrusive rocks do not reach the earth’s surface but - are commonly exposed at the surface by later erosion. - Isometric—a crystal system in which the crystal faces are referred to - three equal intersecting axes at right angles to each other. - Lap—a disc-shaped piece of metal or other material which is - impregnated with diamond dust, or some other cutting or - polishing agent, that is revolved while the gemstone is worked - against it. - Lap plate—a metal plate to which a cutting or polishing lap is - attached, usually by means of a threaded bolt and wing nut. - The lap plate is attached to the shaft which is turned by the - motor under the facet table. - Lapidary—one who practices the lapidary arts; a gem cutter. - Limestone—a sedimentary rock composed mostly of calcium carbonate. - Luster—the appearance of the freshly broken or unweathered surface of - a mineral in reflected light (p. 5). - Main facet—as applied to the standard American brilliant cut, one of - the first eight facets cut on either the crown or pavilion of - a gem (fig. 6). - Matrix—the material in which a specific mineral is embedded; also the - rock to which one end of a crystal is attached. - Metamorphic rock—rock that has been changed from its original state by - heat, pressure, chemical action, or some combination of these - factors. - Millimeter—¹/₁₀ centimeter; approximately ¹/₂₅ inch. - Mineralogy—the science concerned with the study of minerals, including - their occurrence, composition, forms, properties, and - structure. - Monoclinic—a crystal system in which the crystal faces are described - in relation to three intersecting unequal axes, two of which - are at right angles and the third inclined. - Oligocene—a division of geologic time, estimated to be the time from - 40 to 28 million years ago; part of the Cenozoic era. - Opaque—does not transmit light. - Orbicular—containing orbs or spherical or eye-like markings or - structures. - Orthorhombic—a crystal system in which crystal faces are referred to - three unequal intersecting axes at right angles. - Pavilion—the portion of a faceted gem below the girdle (fig. 6). - Pegmatite—a body of coarse-grained intrusive igneous rock, commonly - lens or dike shaped. - Perthitic—a plaid-patterned structure resulting from intermixture of - soda- and potash-rich feldspars. - Phantom crystal—a crystal outline seen within another crystal, mostly - due to entrapping of inclusions during the crystal’s growth. - Pleochroism—the property of transmitting different colors of light in - different crystallographic directions. - Point—a unit of weight equal to ¹/₁₀₀ (0.01) carat. - Porous—containing pores or void spaces. - Precambrian—a division of geologic time, estimated to be all of - geologic time prior to 550 million years ago; the time before - the Paleozoic era. - Preform—a gemstone that has been ground to a rough outline of the - finished shape of a gem. - Rhyolite—a fine-grained extrusive or shallow intrusive igneous rock of - approximately the same composition as granite. - Rough—uncut, not worked by a lapidary, not cut and polished. - Schist—a metamorphic rock that contains an abundance of oriented platy - minerals that enable the rock to be split with relative ease - parallel to the flat surfaces of the platy minerals. - Silicified—replaced by or containing a large amount of quartz or - silica. - Skill facet—a term often used for the pavilion girdle facets of the - standard American brilliant cut (fig. 6). - Specific gravity—the weight in air divided by the loss of weight in - water at a given temperature, or the weight of an object in - air divided by the weight of an equal volume of water; also - called relative density; the most commonly used standard - temperature for this measurement is 4° C. or 39.2° F. - Star facet—one of the eight facets surrounding the table facet of a - standard American brilliant cut (fig. 6). - Step cut—a mode of faceting in which the surface of the gem is covered - by a series of square or rectangular facets; stones thusly cut - are usually square, rectangular, or irregular with straight - sides in outline. - Streak—the color of a mineral when finely powdered; usually determined - by rubbing the mineral against a piece of unglazed porcelain. - Symmetry—the number, location, and balanced arrangement of crystal - faces in reference to the crystallographic axes or other - crystallographic planes or directions. - Synthetic gem—a gemstone manufactured by man that has approximately - the same chemical composition and properties as a natural - gemstone. - Table facet—the large horizontal facet found on the crown of many - gems, often called simply the table (fig. 6). - Tenacity—the resistance of minerals to breakage, described by such - terms as malleable, ductile, sectile, and brittle (p. 6). - Termination—the end of a crystal that is completely enclosed by - crystal faces, the crystal end that is not attached to the - matrix. - Tertiary—a division of geologic time, estimated to be the time from 60 - to 1 million years ago; the Tertiary includes the Paleocene, - Eocene, Oligocene, Miocene, and Pliocene epochs (from oldest - to youngest). - Tetragonal—having four angles; a crystal system in which the crystal - faces are referred to three axes at right angles to each - other, two of which are equal and the third longer or shorter. - Translucent—allowing the passage of light but diffusing it - sufficiently so that objects on the other side cannot be - clearly distinguished. - Transparent—clear, allowing free passage of light so that objects on - the other side can be readily distinguished; opposite of - opaque. - Triclinic—a crystal system in which the crystal faces are referred to - three unequal axes, none of which are at right angles. - Tumbling—a process of polishing irregularly shaped gemstones (p. 17). - Vein—a tabular, irregular, or twisting mineral deposit that is thin in - relation to its length and breadth, usually the result of - solution or hydrothermal activity. - Vitreous—having luster, general appearance, or physical properties - similar to glass. - Vug—an unfilled rock cavity, commonly lined with crystals; may later - become filled by minerals owing to solution or hydrothermal - activity. - - - - - SELECTED REFERENCES - - -Anderson, B. W. (1948) Gem testing: Emerson, New York. - -Baker, C. L. (1935) Metallic and non-metallic minerals and ores -(precious stones), _in_ The geology of Texas, Vol. II, Structural and -economic geology: Univ. Texas Bull. 3401, Jan. 1, 1934, pp. 568-569. - -Barnes, V. E. (1940) North American tektites: Univ. Texas Pub. 3945, -Dec. 1, 1939, pp. 477-582. - -Dake, H. C., Fleener, F. L., and Wilson, B. H. (1938) Quartz family -minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York. - -Ford, W. E. (1932) A textbook of mineralogy (4th ed.): John Wiley and -Sons, Inc., New York. - -Kraus, E. H., and Slawson, C. B. (1947) Gems and gem materials (5th -ed.): McGraw-Hill Book Company, Inc., New York. - -Kunz, G. F. (1892) Gems and precious stones of North America (2d ed.): -Scientific Publishing Company, New York. - -Pough, F. H. (1953) A field guide to rocks and minerals: Houghton -Mifflin Company, Boston. - -Simpson, B. W. (1958) Gem trails of Texas: Granbury, Texas. - -Sinkankas, John (1955) Gem cutting: D. Van Nostrand Company, Inc., -Princeton, New Jersey. - -—— (1959) Gemstones of North America: D. Van Nostrand Company, Inc., -Princeton, New Jersey. - -Smith, G. F. H. (1958) Gemstones (13th ed.), revised by F. C. Phillips: -Methuen and Company, Ltd., London. - -Sperisen, F. J. (1950) The art of the lapidary: The Bruce Publishing -Company, Milwaukee, Wisconsin. - -Sterrett, D. B. (1913) Gems and precious stones, _in_ Mineral resources -of the United States, Calendar Year 1912, Part II, Non-metals: U. S. -Geol. Survey, pp. 1023-1060. - - - - - Plate I - - - [Illustration: A -Gem-quality celestite crystals from Travis County, Texas. Twice natural - size. Lower portion of the crystals is colorless; the tips are dark - blue.] - - [Illustration: B - Opalized wood from the Texas Gulf Coastal Plain. Specimen at left is -rich brown and tan; specimen at right is fossil palm wood and is black, - reddish brown, and white. One-third natural size.] - - - - - Plate II - - - [Illustration: A - Gem-quality garnet crystals and faceted gem from Gillespie County, - Texas. Natural size.] - - [Illustration: B - Labradorite from Brewster County, Texas. Both stones are pale yellow. - One and a half times natural size.] - - - - - Plate III - - - [Illustration: A - Pink microcline crystal from Burnet County, Texas.] - - [Illustration: B -Smoky quartz from Burnet County, Texas. Natural size. Colorless crystal - at center back is included for color comparison.] - - - - - Plate IV - - - [Illustration: Polished agate from gravels of the Rio Grande near -Zapata, Zapata County, Texas. Bands are blue and gray; other inclusions - are brown, yellow, and reddish. One and a half times natural size.] - - - - - Plate V - - - [Illustration: A -Texas tektites (bediasites) showing variety of surface features. Natural - size.] - - [Illustration: B -Topaz crystal from a pegmatite dike near Streeter, Mason County, Texas. -Natural size. Measurements: 1½ by 1⅝ by 3 inches; weight: 194 grams (970 - carats); pale blue; mostly gem quality.] - - - - - Plate VI - - - [Illustration: A - Topaz from stream gravels near Streeter, Mason County, Texas. Natural -size. Left to right: colorless worn pebble; emerald-cut pale-blue topaz, - weight 10 carats; pale-blue worn pebble, weight 205 carats; step out - sky-blue topaz, weight 13 carats; pale-blue worn pebble.] - - [Illustration: B - Tourmaline crystals in schist from Llano County, Texas.] - - - - - Index - - - A - actinolite: 26 - agate: 20, 28, 38 - agatized wood: 27 - allanite: 21 - almandite: 22 - amazonite: 23 - amazon stone: 23 - amber: 18 - amethyst: 25 - Amethyst Hill: 25 - amorphous gemstones: 9 - andalusite: 30 - Arkansas: 19 - Armstrong County: 21 - augite: 18, 28 - - - B - Baringer Hill, Llano County: 21, 26 - baroque pearls and/or stones: 17, 25 - bediasite (tektite): 28-29, 39 - beryl: 18 - Big Bend National Park: 27 - biotite: 23 - Blanco County: 18, 22 - Brazos County: 29 - Brazos River: 25 - Brewster County: 18, 23, 24, 25, 26, 27, 28, 31, 36 - brilliancy: 5 - brilliant cut, standard American: 13, 15, 16 - Brown County: 19 - Burleson County: 29 - Burnet County: 20, 22, 23, 25, 26, 30, 37 - - - C - cabochon gems: 10-12 - Caddo Lake: 25 - carbuncle: 22 - carnelian: 27 - cassiterite: 23, 29 - celestite: 19, 35 - chalcedony: 27 - geodes: 26 - chuck: 15, 17 - citrine: 25-26 - cleavage: 6, 13 - coal: 22 - Coke County: 19 - color: 5 - Colorado River: 25 - Concho River: 25 - crown girdle facets: 16, 17 - crown of gemstone: 15, 16 - crystals: 7-9 - crystal systems: 7 - crytolite: 21 - Culberson County: 22, 23, 25, 26, 31 - culet: 13 - cutting and polishing: 10-17 - cutting lap: 13 - - - D - DeWitt County: 29 - diamond: 19 - saw: 10, 11 - diaphaneity: 5 - dispersion: 6 - dopping: 12, 13 - dop-stick: 12, 15, 17 - dop-wax: 12, 15, 17 - dravite: 30 - durability: 6 - Duval County: 21, 24 - - - E - El Paso County: 22, 30, 31 - emerald cut: 15 - epidote: 19-20 - - - F - facet, kinds of: 13 - main: 16 - skill: 16 - table: 13, 14 - faceted gems and/or stones: 10, 13-17 - Fayette County: 20, 29 - fergusonite: 21 - fire: 5 - Fisher County: 19 - fluorite: 20, 21 - Foard County: 19 - fossil wood: 20-21, 22 - fracture: 6 - Franklin Mountains: 18 - - - G - gadolinite: 21-22 - garnet: 20, 22, 36 - gemstones, by kinds: 18-31 - geodes, celestite: 19 - Georgia: 29 - Gillespie County: 18, 22, 23, 25, 26, 30, 36 - girdle facets: 16 - gneiss: 22 - Gonzales County: 20, 29 - grain: 25 - gram: 7 - graphite: 30 - Grimes County: 29 - grinding: 12 - Guadalupe River: 25 - Gulf Coast: 25 - Gulf Coastal Plain: 18, 20, 22, 35 - - - H - hardness: 6 - Hudspeth County: 18, 20, 22, 24, 25, 26, 27, 28, 31 - - - I - index of refraction: 5 - - - J - jasper: 27-28 - Jeff Davis County: 26, 27, 28, 31 - jet: 22 - - - K - Kinney County: 28 - - - L - labradorite: 23, 36 - Lake Buchanan: 21 - Lampasas County: 19 - lap plate: 13 - Lavaca County: 29 - Lee County: 20, 29 - lignite: 22 - Live Oak County: 21 - Llano County: 18, 19, 20, 21, 22, 23, 25, 26, 30, 31, 40 - Llano River: 25 - luster: 5 - - - M - Madeira topaz: 29 - Mason County: 20, 22, 23, 26, 29, 30, 39, 40 - Massachusetts: 29 - Maverick County: 18 - microcline: 20, 21, 23-24, 29, 37 - Mohs scale of hardness: 6 - moonstone: 28 - Mount Bonnell: 19 - muscovite: 23 - - - N - natural glass: 18, 24, 28 - Needle Peak, Presidio County: 27 - nivenite: 21 - Nolan County: 19 - Nueces River: 25 - - - O - obsidian: 24, 29 - Oklahoma: 29 - opal: 20, 24 - opalized wood: 35 - orbicular jasper: 28 - ounce: 7 - - - P - Packsaddle Mountain: 23 - Packsaddle schist: 30 - palm wood: 21, 35 - Palo Duro Canyon: 21 - pavilion: 13, 16 - facets: 16 - girdle facets: 16, 17 - pearl: 24-25 - pegmatites and/or pegmatite dikes: 18, 20, 21, 22, 23, 26, 29, 39 - petrified wood: 26 - phantom crystals: 26 - pistacite: 20 - pleochroism: 5 - point: 7 - polishing: 17 - lap: 13, 16 - preformed stone: 16 - preforming: 15 - Presidio County: 20, 22, 24, 25, 26, 27, 28 - properties of gemstones: 5-7 - - - Q - quartz: 20, 21, 23, 25-28, 29, 30 - smoky: 38 - - - R - radioactive elements: 22 - radioactivity of gadolinite: 21 - rarity: 6 - Reeves County: 26, 27 - Rio Grande: 25 - gravels of: 21, 27, 38 - Valley: 27 - rock crystal: 26 - rose quartz: 26 - - - S - Sabine River: 25 - sanding: 12 - sanidine: 28 - sawing: 10 - scheelite: 20 - schorl: 30 - size: 7 - “skill” facets: 16 - “slab” of gem materials: 11 - Smithsonian Institution: 21 - smoky quartz: 23, 26, 37 - smoky topaz: 29 - Spanish topaz: 29 - specific gravity: 7 - spinel: 18, 28 - star facets: 17 - step cut: 15 - streak: 6 - synthetic gems: 7 - - - T - table facet: 13, 15 - tektite (bediasite): 28-29, 39 - tenacity: 6 - thorogummite: 21 - topaz: 23, 26, 29-30, 39, 40 - quartz: 29 - tourmaline: 26, 29, 30-31, 40 - Town Mountain, Llano County: 26, 30 - transparency: 6 - Travis County: 19, 35 - Trinity River: 25 - tumbled gems: 17 - turquoise: 31 - - - U - U. S. National Museum: 30 - - - V - value of gemstones: 6, 7 - Val Verde County: 24 - valverdites: 24 - Van Horn, Hudspeth County: 27 - - - W - Walker County: 29 - Washington County: 20 - Webb County: 21 - weight, units of: 7, 25 - - - Z - Zapata County: 38 - - - - - Transcriber’s Notes - - ---Silently corrected a few typos. - ---Renumbered figures 6 and 7 (and references to them) to correspond to - their order in the printed book. - ---Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - ---In the text versions only, text in italics is delimited by - _underscores_. - - - - - - - -End of the Project Gutenberg EBook of Texas Gemstones, by Elbert A. 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Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. - -Most people start at our Web site which has the main PG search -facility: www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. - diff --git a/old/60070-0.zip b/old/60070-0.zip Binary files differdeleted file mode 100644 index 7cd7d3d..0000000 --- a/old/60070-0.zip +++ /dev/null diff --git a/old/60070-8.txt b/old/60070-8.txt deleted file mode 100644 index 7720624..0000000 --- a/old/60070-8.txt +++ /dev/null @@ -1,2940 +0,0 @@ -The Project Gutenberg EBook of Texas Gemstones, by Elbert A. King, Jr. - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Texas Gemstones - -Author: Elbert A. King, Jr. - -Release Date: August 6, 2019 [EBook #60070] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK TEXAS GEMSTONES *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - - - - - BUREAU OF ECONOMIC GEOLOGY - The University of Texas at Austin - Austin, Texas 78712 - - JOHN T. LONSDALE, _Director_ - - - Report of Investigations--No. 42 - - - - - Texas Gemstones - - - By - Elbert A. King, Jr. - - February 1961 - - _Second Printing--February 1963 - Third Printing--September 1972 - Fourth Printing--March 1983 - Fifth Printing--August 1991_ - - - - - Contents - - - Page - Introduction 5 - Properties of gemstones 5 - Crystals 7 - Cutting and polishing of gemstones 10 - Cabochon gems 10 - Faceted gems 13 - Tumbled gems 17 - Texas gemstones 18 - Amber 18 - Augite 18 - Beryl 18 - Celestite 19 - Diamond 19 - Epidote 19 - Fluorite 20 - Fossil wood 20 - Gadolinite 21 - Garnet 22 - Jet 22 - Labradorite 23 - Microcline 23 - Obsidian 24 - Opal 24 - Pearl 24 - Quartz 25 - Crystalline varieties 25 - Amethyst 25 - Citrine 25 - Rock crystal 26 - Rose quartz 26 - Smoky quartz 26 - Cryptocrystalline varieties 27 - Chalcedony 27 - Agate 27 - Agatized wood 27 - Carnelian 27 - Jasper 27 - Sanidine 28 - Spinel 28 - Tektite (bediasite) 28 - Topaz 29 - Tourmaline 30 - Turquoise 31 - Glossary 32 - Selected references 34 - Index 41 - - - - - Illustrations - - - Figures-- Page - 1. Typical crystal form of three common Texas gemstones 9 - 2. Variations of the cabochon cut 10 - 3. Diamond saw 11 - 4. Cabochon properly attached to dop-stick 12 - 5. Cabochons at various stages of cutting and polishing 12 - 6. Nomenclature of the standard American brilliant cut 13 - 7. Facet table 14 - 8. Grinding the table facet on a rough stone 15 - 9. Stone dopped to table facet 15 - 10. Preformed stone dopped to table facet 16 - 11. Proper sequence of cutting of the pavilion facets 16 - 12. Proper placing of pavilion girdle facets 17 - 13. Proper sequence of cutting of crown facets 17 - 14. Common crystal form of Travis County celestite 19 - 15. Common crystal form of fluorite 20 - 16. Crystal faces on microcline specimen shown in Plate III 23 - 17. Common crystal form of spinel 28 - 18. Crystal faces on topaz crystal shown in Plate V 29 - 19. Cross section through irregularly colored stone 30 - 20. Common crystal form of Llano County tourmaline 31 - - - Plates-- Page - I. A, Gem-quality celestite crystals from Travis County. B, - Opalized wood from the Texas Gulf Coastal Plain 35 - II. A, Gem-quality garnet crystals and faceted gem from Gillespie - County. B, Labradorite from Brewster County 36 - III. A, Pink microcline crystal. B, Smoky quartz. Both from Burnet - County 37 - IV. Polished agate from gravels of the Rio Grande near Zapata, - Zapata County 38 - V. A, Texas tektites (bediasites). B, Topaz crystal from a - pegmatite dike near Streeter, Mason County 39 - VI. A, Topaz from stream gravels near Streeter, Mason County. B, - Tourmaline crystals in schist from Llano County 40 - - - Table 1. Properties of some common Texas gem minerals 8 - - - - - Texas Gemstones - - - ELBERT A. KING, JR. - - - - - INTRODUCTION - - -Throughout history man has sought stones and minerals for personal -adornment and ornamentation. Stones and minerals that are sufficiently -beautiful, durable, and rare are known as gemstones. A gemstone with -only one of these qualities is less desirable than one with all three. -For example, a stone with rich color but not sufficiently durable to -withstand daily wear in rings finds little favor as a gemstone except in -brooches or pins where the stone is relatively safe from abrasion. -Likewise, a stone that is beautiful and durable may be of little -interest as a gemstone because it is commonly found in great quantities. -To be valued highly, gemstones must be beautiful to the eye, durable -enough to withstand wear, and rare enough so that they are not easily -obtained. - - - - - Properties of Gemstones - - -The beauty of gemstones is mostly dependent on their color, diaphaneity, -brilliancy, luster, and fire. Any one or a combination of these -properties render stones desirable as gems. - -Color is very important in many gemstones. The color of transparent -varieties should be distinct enough to be pleasing to the eye, yet not -so dark as to appear black or opaque. It is generally more desirable -that the gemstone be of even color and not appear "patchy" or -"streaked." However, some opaque or translucent stones such as agate owe -their popularity chiefly to the variety of colors and designs within a -single piece. Some transparent gemstones exhibit different colors when -viewed in different directions. For example, some fine blood-red rubies -appear brownish when viewed in a particular direction. The gemstone -should be cut so that its finest color is most prominently displayed. -This ability of some gemstones to exhibit different colors when viewed -in different directions is called pleochroism. - -Diaphaneity is the relative ability of stones to transmit light. -Diaphaneity is described by terms such as transparent, translucent, and -opaque. Transparency is highly desirable in stones such as diamond that -are commonly facet-cut to reflect light. The gemstone should be water -clear and free from inclusions and cracks so that it transmits light -freely, but there are stones that do not exhibit this property that are -prized as gemstones. For example, turquoise may appear to be completely -opaque and not transmit any light, but it is sought for its fine blue -color. - -The brilliancy of gemstones is largely dependent on their index of -refraction. The index of refraction is a measure of the ability of a cut -gemstone to "bend" light rays and reflect them from the bottom facets -back through the top of the stone. Of course, brilliancy is not noted in -opaque or faintly translucent stones. The index of refraction of -gemstones is expressed numerically. Air is the reference medium and is -assigned an index of refraction of 1.00. Other substances are assigned -values relative to that of air, for example, water, 1.33; topaz, 1.62; -diamond, 2.42. The higher the index of refraction, the more brilliant -will be the gemstone if it is properly cut and polished. - -Luster is the appearance of the mineral on a fresh surface in reflected -light; it is divided into two major categories, metallic and -non-metallic. Most gemstones have non-metallic luster and are described -by terms such as vitreous or glassy, resinous, waxy, greasy, and pearly. - -The fire, or ability of gemstones to show flashes of different colors of -light, is dependent upon a property called dispersion. The amount of -dispersion is the extent to which the gemstone is able to separate -ordinary white light into its component colors. The dispersion of -gemstones can also be expressed numerically but for purposes of this -publication will be referred to as low, moderate, or high. Diamond is a -common gemstone that has high dispersion. - -A gemstone's durability is primarily dependent upon its hardness. The -Mohs scale of hardness, given below, is most commonly used for gemstones -and other minerals. - - _Mohs Scale of Hardness_ - 1. Talc - 2. Gypsum - 3. Calcite - 4. Fluorite - 5. Apatite - 6. Orthoclase feldspar - 7. Quartz - 8. Topaz - 9. Corundum - 10. Diamond - -On this scale, the higher numbers are the harder minerals. Mohs is a -relative, not an absolute scale. Therefore, it should not be assumed -that diamond is ten times harder than talc because actually diamond is -very many tens of times harder than talc. However, a particular mineral -is harder than any other mineral with a lesser number, and the scale is -very convenient to use. Gemstones mounted in rings should have a -hardness of at least seven on the Mohs scale, or the stones may become -scuffed and scratched after a relatively short period of wear. Gemstones -mounted in pins and brooches can be of softer material as they are not -usually subjected to abrasion and rough treatment. - -The tendency of some minerals to split with relative ease in particular -directions along planes is called cleavage. Cleavage is also a factor -determining the durability of gemstones. Some gemstones do not exhibit -this tendency at all, whereas others cleave in several directions. The -number of cleavages is always the same in any one mineral, and the -direction of cleavages is constant in relation to the crystal structure -of any one mineral or gemstone. It is apparent that of stones having the -same hardness, the ones lacking cleavage or having the lesser number of -good cleavage directions are the most durable. - -Some stones, such as jade and agate, owe their durability to their -compact fibrous structure, which makes them very tough and durable even -though they are not especially hard. - -Several other properties of gemstones, although not always contributing -to the beauty or desirability of gemstones, are useful in identifying -uncut specimens. - -Streak is the color of the mineral when finely powdered or, for softer -minerals, the color obtained by rubbing the mineral against a piece of -unglazed porcelain or tile. The color of a mineral's streak is commonly -different from the unpowdered specimen. - -Fracture is the kind of surface obtained when the mineral is broken in a -direction that is not a cleavage direction. Fracture surfaces are -described by such terms as conchoidal (like the fracture of glass), -subconchoidal, splintery, even, and uneven. - -Tenacity is the resistance of a mineral to breakage. Brittle minerals -break relatively easily on impact. Malleable minerals, such as gold, may -be flattened under a hammer into very thin sheets without breaking. -Sectile minerals may be cut with a knife without powdering. Most -gemstones, even diamond, are brittle. - -It is only natural to value most those gemstones that are not common or -easy to obtain. Emerald owes its longstanding popularity to its fine -green color, but tourmaline is sometimes found in colors that very -closely approach that of emerald and yet sells for considerably less -because it is so much more common. - -Rarity is not the only factor affecting the value of gemstones. Freedom -from internal imperfections, quality of cutting, color, and size must -also be considered in cut and polished gemstones. Internal -imperfections, such as inclusions and cracks, detract from the -appearance of gemstones and interfere with the passage of light between -the facets; consequently, gemstones containing these imperfections are -not valued as highly as those without them. Poor cutting or polishing -detract from the beauty and thus from the value of gemstones. Unpopular -or poor color commonly causes gemstones to be less valuable. Rich green -emeralds are exceedingly prized, whereas very pale green emeralds are -relatively inexpensive. Diamonds that have the least hint of yellow are -never valued as highly as pure colorless, pink, or blue stones. Few -persons find the yellowish color attractive, unless it is a vivid canary -yellow. - -Size is important in determining the value of gemstones but not as -important as perfection. A badly flawed gemstone of large size may be -worth only a slight fraction of the value of a smaller perfect one. -Gemstone size is usually measured in carats, a unit of weight, although -millimeter size is sometimes used. Five carats is equal to 1 gram and -approximately 28-1/3 grams is equal to 1 ounce avoirdupois. One -one-hundredth (0.01) of a carat is called a point, and this term is -often used, especially pertaining to very small gemstones. - -The term used to compare the relative weights of minerals and gemstones -is specific gravity, which is expressed numerically in relation to -water. Water is assigned the value of 1.00. Therefore, at a given -temperature a gemstone having a specific gravity of 2.00 is twice as -heavy as an equal volume of water. A 1-carat sapphire (specific gravity -about 4.00) will be smaller than a 1-carat amethyst (specific gravity -about 2.65) because the heavier material will occupy less volume to have -the same weight. - -A summary of properties helpful in identification of common Texas gem -minerals is given in Table 1. - -Comparatively recently in the history of gemstones, man has succeeded in -the production of synthetic gems that have properties closely -approaching those of many natural gemstones. To the untrained eye some -synthetic gems may appear identical to natural stones, but synthetic -gems can be detected with little difficulty by a properly equipped -expert. Although most synthetic gems are inexpensive, their manufacture -has not adversely affected the value of natural gemstones but instead -has increased the demand for fine natural gems. - - - - - Crystals - - -Gemstones that have an orderly internal molecular arrangement are -referred to as crystalline. This internal order is commonly reflected in -the external shape of "rough" or uncut gemstones. The resultant shape is -a polyhedral solid bounded by planes and called a crystal. Well-formed -crystals are formed in nature only under relatively ideal conditions of -temperature, pressure, and space. The specific temperatures and -pressures involved vary with different minerals, but most crystals need -space in which to form so that their "growth" is not impaired by -surrounding rocks and minerals. However, some minerals, such as garnet -and tourmaline, can grow in metamorphic rocks by recrystallization of -minerals in the metamorphic rocks. The size of crystals varies from -microscopic to tens of feet. Any one mineral usually has one or two -typical crystal forms or arrangements of plane surfaces that aid greatly -in the identification of the mineral when it occurs in good crystals -(fig. 1). Frequently gemstones are found as abraded stream-rolled -pebbles, fragments, or masses that do not show crystal form. Crystals of -the same mineral from different locations commonly show somewhat -different crystal forms owing to slight differences in composition or -conditions of formation. Mineralogists and crystallographers classify -crystals by the symmetry that they exhibit. The crystal systems are (1) -isometric or cubic, (2) tetragonal, (3) hexagonal, (4) orthorhombic, (5) -monoclinic, and (6) triclinic. A complete description of the -classification of crystals can be found in almost any mineralogy text -(see Selected References, p. 34). - - Table 1. Properties of some common Texas gem minerals. - MINERAL COMPOSITION HARDNESS SPECIFIC INDEX OF COMMON - GRAVITY REFRACTION COLORS IN - TEXAS - - Amber fossil resin 2.0-2.5 1.05-1.10 about 1.54 brown, yellow - Augite CaMgSi_2O_6 5.0-6.0 3.2-3.6 1.60-1.71 greenish - brown, black - Beryl Be_3Al_2(SiO)_6 7.5-8.0 2.63-2.80 1.56-1.60 pale blue, - colorless, - greenish - Celestite SrSO_4 3.0-3.5 3.95-3.98 1.62-1.63 colorless, - blue - Epidote HCa_2(Al, 6.0-7.0 3.25-3.50 1.72-1.77 yellowish - Fe)_3Si_3O_1_3 green, - brownish - green - Fluorite CaF_2 4.0 3.0-3.25 1.434 colorless, - violet, - yellow, green - Garnet Fe_3Al_2(SiO_4)_3 about 7.5 4.25 about 1.83 red, deep - (Almandite) red, - brownish red - Labradorite NaAlSi_3O_8 50% to 6.0-6.5 about 2.6 about 1.56 yellowish, - 30% CaAlSi_3O_8 50% grayish - to 70% - Microcline KAlSi_3O_8 6.0-6.5 2.54-2.57 1.52-1.53 pink, red, - bluish, - greenish, - white - Obsidian volcanic glass 5.0-5.5 2.3-2.5 1.45-1.53 dark gray, - black, - brownish - Opal SiO_2nH_2O 5.5-6.5 1.9-2.3 1.43 white, pink, - bluish, - brown, gray - Quartz SiO_2 7.0 2.65-2.66 1.544-1.553 colorless, - (Crystalline) violet, - yellow, brown - Tektite natural glass 5-6 2.33-2.44 1.48-1.52 dark brown, - (Bediasite) greenish - brown - Topaz Al_2(FOH)_2SiO_4 8.0 3.4-3.6 1.60-1.63 colorless, - bluish, sky - blue - Tourmaline H_9Al_3(BOH)_2Si_4O_1_9 7.0-7.5 2.98-3.20 1.62-1.64 black, dark - brown - -Some gemstones, such as opal and obsidian, never occur as crystals owing -to a lack of internal structural order. Such gemstones are termed -amorphous, or without form. Amorphous gemstones mostly occur in nature -as irregular lumps or masses, cavity fillings, or veins. - - [Illustration: Fig. 1. Typical crystal form of three common Texas - gemstones.] - - GARNET - TOURMALINE - QUARTZ - - - - - CUTTING AND POLISHING OF GEMSTONES - - -There are two types of widely used gemstone cuts. Opaque or figured -gemstones are usually cut with a rounded upper surface and a flat or -rounded back. A stone cut in this fashion is termed a cabochon or is -said to be cabochon cut. There are several variations of this mode of -cutting (fig. 2). Precious opal, agate, jade, star sapphire, and fossil -wood are some of the stones that are cut mostly as cabochons. -Transparent gemstones are usually cut with many plane polished surfaces. -Such stones are called faceted, and the process of cutting and polishing -these stones is called faceting. Emerald, diamond, topaz, and garnet are -examples of gemstones that are commonly seen as faceted stones. - - [Illustration: Fig. 2. Variations of the cabochon cut. Left to right: - double cabochon; flat cabochon; simple cabochon; hollow cabochon.] - -The cutting of gemstones, although sometimes tedious and time consuming, -is not especially difficult or complex. However, like most arts and -crafts, technique and ability should improve with practice and -experience. There are currently many amateur gem cutters in Texas. A -complete set of equipment necessary to cut cabochon stones may be -purchased for as little as $50.00 or $60.00. Most amateur cabochon -cutters have equipment that cost less than $100.00 which enables them to -do very fine work on many gem materials. Facet cutting requires more -precise equipment, and a complete array of such usually costs more than -$100.00, although less expensive equipment can be obtained. The -beginning gem cutter or lapidary who is willing to assemble and make -some of his own equipment can reduce his initial expenses considerably. - - - - - Cabochon Gems - - -The procedures listed herein for gem cutting do not apply to all -gemstones. Stones that are especially brittle, soft, or difficult to -polish require additional procedures or special techniques. Many -lapidaries may deviate from these procedures. Some of the steps of -cutting and polishing are merely matters of personal opinion and vary -somewhat from cutter to cutter. There are several detailed texts on the -art of gem cutting; the descriptions herein are designed to give the -reader only a general idea of the procedures and techniques involved. - -The cutting and polishing of cabochons require several steps. The -initial step is sawing. Assuming that the rough gem material is large -enough to be sawed (larger than about half an inch in diameter), it is -clamped into the carriage of a diamond saw (fig. 3) and cut into slices -about 3/8-inch thick. The blade of the saw is mild steel that has been -impregnated with diamond dust around the edge, hence the name diamond -saw. The blade is rotated rapidly, and the material to be cut is "fed" -to the blade by a sliding carriage on which the gem material is clamped. -The extreme hardness of the diamond dust in the edge of the blade -enables the saw to cut through several inches of gem material in a few -minutes. The lower portion of the saw blade is immersed in a mixture of -kerosene and oil, and the rotating saw blade carries with it some of the -kerosene-oil mixture; this acts as a coolant and lubricant for both the -saw blade and the material being cut. Without this lubricant, the heat -generated by sawing would shatter most gem materials and also damage the -saw blade. As this "slicing" or sawing of the material usually takes -several minutes, a weight and pulley are generally used to give the gem -material the necessary pressure against the saw blade. When cut through, -the "slab" of gem material falls into the kerosene-oil mixture at the -bottom of the saw or onto a special platform that cushions its fall. - - [Illustration: Fig. 3. Diamond saw.] - - Motor - Clamp - Diamond-charged blade - Carriage - Stone - Weight - -After being sawed, the slab of gem material is examined, and the -location and size of the stones to be cut from the slab are determined. -The desired outline of the shape of the gem to be cut is marked on the -slab with a pointed piece of aluminum rod; ordinary pencil marks are not -used because they wear away too quickly in the cutting process. Once the -area from which the gem is to be cut has been selected and the outline -of the gemstone has been marked on the slab, the excess material is -trimmed away by a smaller diamond saw known as a trim-saw. In some slabs -the excess material can be broken and "nibbled" away with a strong pair -of pliers. - -The remaining portion of the stone is usually held by hand and ground to -the desired shape using the previously scribed mark as a guide. This is -done using a relatively coarse-grained (about 150 grit) specially made -carborundum grinding wheel. - -Now that the desired outline has been obtained, the stone is firmly -affixed to a slender wooden or hollow aluminum dop-stick (fig. 4). The -process whereby the stone is attached to the dop-stick with a specially -compounded jeweler's wax is called dopping. The dop-wax is heated over -an alcohol lamp or candle flame until it is soft and pliable and is then -spread around on the end of the dop-stick and formed into a mass about -the right size and shape to fit the back of the gemstone. The stone is -likewise heated, and the wax is applied to the back of the stone while -both wax and stone are hot. Upon cooling, the wax firmly fixes the stone -to the dop-stick. The dop-stick allows the lapidary to have firm control -of the stone during all later stages of cutting and polishing. - - [Illustration: Fig. 4. Cabochon properly attached to dop-stick.] - - CABOCHON - DOP-WAX - DOP-STICK - -The top of the dopped gemstone is worked against the coarse carborundum -grinding wheel until it is a rough approximation of the desired shape. -The stone is then worked against a much finer-grained (about 220 grit) -grinding wheel to remove the irregularities left by the coarse grinding -and to further smooth and shape the surface of the gemstone. At all -times while grinding, a small flow of water should be directed on the -grinding wheel to keep the stone cool. Grinding on the stone for even a -few minutes without cooling may result in the shattering of the gemstone -because of heat created by friction of the stone against the grinding -wheel. If the lapidary keeps the surface of the grinding wheel wet, -there is little chance of damaging most gem materials. - -The next phase of cabochon cutting and polishing is sanding. The -gemstone is worked against two sanding drums of different grit size. -This sanding can be done with the sandpaper surface either wet or dry, -as needed or as preferred by the lapidary. However, great care should be -exercised during sanding so that the stone is not overheated. -Overheating can easily occur whether the sandpaper is used wet or dry. -As in grinding, sanding is first done on coarser grit paper (about 300 -grit) and last on finer paper (about 600 grit). It is in the sanding -process that the first hint of polish is noted on the surface of the -stone. After sanding, the gemstone should have perfect form with no -surface irregularities, a very finely textured surface, and only very -minor scratches left from sanding. The gemstone is now ready to be -polished. - - [Illustration: Fig. 5. Cabochons at various stages of cutting and - polishing. Left to right: trimmed from slab: ground to outline; after - rough grinding; after sanding; polished.] - -At this point the procedure depends on the nature of the gemstone being -polished. Most gem materials are worked against a buffing wheel that is -impregnated or saturated with a mixture of some polishing compound and -water. A soft felt buffing wheel with cerium oxide as the polishing -agent is used for many materials. The mixture of cerium oxide and water -is usually applied to the buffing wheel with a small brush. The lapidary -should once more be careful not to overheat the stone. If the stone -becomes too hot to hold to the underside of the cutter's wrist, it -should be permitted to cool for a few seconds before continuing. After -polishing on the buffing wheel, the gemstone should have a fine, high -polish and be free of any scratches or surface irregularities. The -finished gemstone is removed from the dop-stick by heating the dop-wax -and pulling the stone loose. Any excess wax that hardens again before it -can be removed from the stone by hand can be dissolved away by rubbing -with an acetone-soaked cloth. Figure 5 illustrates the desired -appearance of the gemstone at the end of each of the steps of cutting -and polishing. - - - - - Faceted Gems - - -The principles involved in faceting are about the same as those in the -cutting of cabochons, but the equipment and technique are considerably -different. The equipment required for the facet cutting of gemstones is -built into or attached to a small specially constructed table (fig. 7), -and the unit is commonly called a facet table. Most faceted gemstones -are cut to obtain the largest flawless stone possible from the rough -material. Therefore, one of the first and most important steps for the -lapidary is to decide how the stone is to be cut from the rough crystal -or pebble. The colors that can be obtained from the gemstone must also -be considered, and the cutting of the stone oriented so that its best -color is displayed. The lapidary also selects the orientation of the -stone in relation to the cleavage or cleavages. It is difficult or -impossible to polish facets of gemstones that are cut parallel to a good -cleavage direction. - - [Illustration: Fig. 6. Nomenclature of the standard American brilliant - cut.] - - TOP VIEW - SIDE VIEW - Star facet - Crown main facet - Crown girdle facet - Pavilion girdle facet - Pavilion main facet - TABLE - CROWN GIRDLE - PAVILION - CULET - BOTTOM VIEW - -Once the orientation of the gemstone to be cut from the rough material -has been determined, the stone is dopped onto a special metal dop-stick -that fits into the chuck of the facet head. The chuck is tightened so -that the position of the stone on the end of the arm of the facet head -is firmly fixed, and the facet head is adjusted so that the first facet -that is cut is the horizontal, top facet of the stone or table facet -(fig. 6). The table facet is cut by grinding the gemstone on a flat -cutting lap that is diamond impregnated (fig. 8). By minor adjustments -of the facet head, the lapidary can precisely control the location of -the table facet. As soon as the table facet has been ground to the -proper size, the cutting lap is removed from the lap plate, and the -polishing lap is secured in place. Many different kinds of polishing -laps and polishing compounds may be used depending on the properties of -the material being polished. However, one lap and one polishing compound -are usually sufficient for each gem variety. After the polishing lap is -secured to the lap plate, the lapidary adjusts the facet head so that -the stone is in exactly the same position relative to the lap that it -was during the cutting of the table facet. The polishing lap is run wet -or damp with water, as is the cutting lap, and small amounts of the -polishing compound are applied to the surface of the lap while the facet -is being polished. The minor scratches left by the cutting process are -gradually removed, and a fine lustrous polish develops on the facet. It -is especially important to take care in achieving a perfect polish on -the table facet, as this facet occupies a large area of the crown of the -gemstone. When the cutting and polishing of the table facet are -completed, the gemstone is still rough or uncut in all portions except -for this single, large, polished surface. - - [Illustration: Fig. 7. Facet table.] - - Water - Light - Adjusting ring - Post - Arm - Chuck - Stone - Abrasives - DIAMOND DUST - CALCIUM OXIDE - LANDE-A - - [Illustration: Fig. 8. Grinding the table facet on a rough stone.] - - CHUCK - DOP-STICK - DOP-WAX - STONE - LAP - -The gemstone is then removed from the dop-stick by melting the dop-wax -and is dopped once more so that the plane of the polished table facet is -perpendicular to the axis of the chuck and arm of the facet head (fig. -9). Great care should be taken by the lapidary to insure that the table -of the stone is exactly perpendicular to this axis, or the proper -placing of the later facets on the stone may become very difficult. - - [Illustration: Fig. 9. Stone dopped to table facet.] - - TABLE FACET - DOP-WAX - STONE - DOP-STICK - -Once the stone has been properly dopped to the table facet, the lapidary -is ready to proceed with the cutting of the outline of the stone. If it -is to be a brilliant cut, the stone is ground perfectly round in -outline; if it is to be an emerald or step cut, it is shaped so that it -is square or rectangular in outline. This process is called preforming. -The arm of the facet head is lowered on the post until it is horizontal, -and the stone is worked against the cutting lap until the desired shape -is obtained. When the preforming process is completed, the stone should -have the desired outline of the finished gem (fig. 10). - - [Illustration: Fig. 10. Preformed stone dopped to table facet.] - - DOP-WAX - STONE - DOP-STICK - -The lapidary is now ready to proceed with the cutting of the pavilion of -the stone. The arm of the facet head is raised to the proper angle for -cutting the main pavilion facets. The angle at which the main facets are -cut is very critical in determining the beauty of the finished stone. -The required angle at which these facets must be cut varies with the -refractive indices of the different varieties of gem minerals. If the -facets are not cut at exactly the proper angle, light entering the top -or crown of the gemstone can pass completely through the stone, instead -of being reflected back out of the crown facets. The result is a dull, -lifeless stone that appears to have a "hole" or "fish-eye" in the -center. Stones that are cut in this manner are greatly reduced in value. -The angle at which the facets are cut is controlled by the adjustment of -the height of the arm of the facet head on the post. The lapidary will -continually adjust this height, because the angle between the arm and -the surface of the lap changes slightly as the facet is ground down to -its proper place and size. - - [Illustration: Fig. 11. Proper sequence of cutting of the pavilion -facets. Left to right: four main facets; all eight main facets; half of - the pavilion girdle facets; completed pavilion.] - -The standard American brilliant cut will be used as an example of facet -cutting. Procedure for all other cuts is essentially the same to this -point. After the eight main pavilion facets have been cut, the cutting -angle is changed a few degrees, the arm of the facet head rotated -slightly, and the sixteen pavilion girdle facets or "skill" facets, as -they are often called, are cut (fig. 11). The pavilion girdle facets -should meet exactly in the center of the main facets at the girdle of -the stone. The pavilion girdle facets should neither overlap, nor should -there be any space between them (fig. 12). After the pavilion girdle -facets are cut, the cutting of the pavilion of the gemstone is -completed. The facets are then polished on the polishing lap at the same -angles and in the same order as they were cut, and the pavilion of the -gem is completely finished. - -The stone is then removed from the dop-stick by melting the dop-wax and -is re-dopped to the pavilion facets so that the crown of the stone is -now exposed for cutting. Before the lapidary proceeds with the cutting -of the crown, it is necessary that the stone be perfectly centered on -the dop-stick and that the plane of the table facet be perpendicular to -the dop-stick and to the axis of the arm of the facet head. The eight -main facets are cut first, with numerous adjustments being made by the -lapidary to insure that the proper angle is maintained (fig. 13). Then -the cutting angle is changed a few degrees, the arm of the facet head -rotated slightly, and the crown girdle facets are cut. The crown girdle -facets are placed very similarly to the pavilion girdle facets except -that they are shorter. The crown girdle facets should be joined in -exactly the same way as the pavilion girdle facets. When these facets -are properly cut, the cutting angle is again changed, the arm rotated, -and the eight star facets are cut. This completes the cutting of the -crown of the stone. The cutting lap is removed from the lap plate, and -the polishing lap is secured into place. The facets are carefully -polished in the same order that they were cut. After the last star facet -has been polished, the stone is removed from the dop-stick. Any excess -dop-wax is removed from the stone by means of a solvent, and the full -beauty of the finished gem is revealed. - - - - - Tumbled Gems - - -One other method of finishing gemstones that deserves mention is -tumbling. "Baroque" or "free-form" stones are produced in this manner. -Loose pebbles or pieces of gem materials left over from other cutting -processes are placed in a small barrel or specially constructed box with -loose carborundum grit. The barrel is turned by means of a small motor, -and the abrasion of the pebbles and grit against each other tends to -round the pebbles and give them a finely pitted surface. Progressively -finer and finer carborundum grit is used, and eventually a polishing -compound. The result is several pounds of well-polished gem pebbles of -various shapes and sizes. These baroque stones have found recent favor -in costume jewelry of modern design. The tumbling process is rather -slow, commonly requiring several days or weeks. However, little effort -is involved on the part of the lapidary, and, consequently, the cost of -most tumbled or baroque stones is quite modest. Only gem material that -is unsuitable for cutting in other manners should be finished in this -way. - - [Illustration: Fig. 12. Proper placing of the pavilion girdle facets. - Left: facets not joined. Center: facets overlapped, joined too high. - Right: correct placing.] - - Stone - Dop-wax - Dop-stick - Chuck - -[Illustration: Fig. 13. Proper sequence of cutting of the crown facets. - Left to right: four main facets; all eight main facets; half of the - crown girdle facets; completed crown.] - - - - - TEXAS GEMSTONES - - - - - Amber - - - _Composition_: fossil resin. _Crystal system_: amorphous. _Hardness_: - about 2.0 to 2.5. _Specific gravity_: variable, from 1.05 to 1.10. - _Luster_: resinous. _Color_: brown, yellow, red, orange, and white. - _Streak_: white to yellowish to gray. _Cleavage_: none. _Fracture_: - conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to - translucent. _Refractive index_: variable, about 1.54. Burns with a - sweet, piney odor. - -Rich brown to yellowish amber has been found near Eagle Pass, Maverick -County, in Cretaceous coal and on Terlingua Creek, Brewster County. -Although much of this material is translucent and the quality suitable -for lapidary purposes, the pieces are seldom more than a fraction of an -inch in diameter. - -Occasional finds of poor quality brownish amber have been reported from -the Tertiary formations of the Gulf Coastal Plain, but thus far no gem -quality material has been found. - -The softness of amber limits its use to brooches, necklaces, and other -jewelry that is relatively safe from abrasion. - - - - - Augite - - - _Composition_: CaMgSi_2O_4; may also contain iron, aluminum, and - sometimes titanium. _Crystal system_: monoclinic. _Hardness_: 5 to 6. - _Specific gravity_: 3.2 to 3.6. _Luster_: vitreous to dull. _Color_: - dark greenish brown and greenish black. _Streak_: light grayish green. - _Cleavage_: two directions, poor. _Fracture_: conchoidal to uneven. - _Tenacity_: brittle. _Diaphaneity_: opaque to translucent. _Refractive - index_: variable, about 1.60 to 1.71. - -Augite of gem quality occurs near Eagle Flat, Hudspeth County, Texas. -Although this material is very dark greenish brown and not commonly -thought of as a gemstone, lapidaries have used it to fashion black -faceted stones and cabochons that resemble obsidian. Most of the augite -occurs as loose pieces and crystal fragments that have weathered out of -nearby igneous rocks; the augite can also be found in situ in the -igneous rocks. - -Specimens and pieces of cutting quality 1 inch in diameter are common, -and fragments over 2 inches in diameter have been found. The augite is -associated with black spinel and some dark gray to black pieces of -natural glass. Although the faceted and cabochon-cut stones are not -particularly attractive, some of the larger pieces of augite might be -utilized for carving. - - - - - Beryl - - - _Composition_: Be_3Al_2(SiO)_6. _Crystal system_: hexagonal. - _Hardness_: 7.5 to 8.0. _Specific gravity_: 2.63 to 2.80. _Luster_: - vitreous. _Color_: pale blue, blue, green, yellow, brownish, pink, and - colorless. _Streak_: white. _Cleavage_: one direction, very imperfect. - _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: - transparent to subtranslucent. _Refractive index_: 1.56 to 1.60. - _Dispersion_: low. - -Gem-quality beryl has not been reported in Texas. A discussion of beryl -is included herein because the writer believes it likely that beryl of -gem quality will be found in Texas as a result of future investigations -and exploration. - -Beryl crystals have been found in pegmatite dikes in Llano, Blanco, and -Gillespie counties. These crystals are commonly several inches long and -exceed 1 inch in diameter but are very badly fractured. Most of the -beryl crystals do not approach gem quality and are entirely unsuitable -for any lapidary use. The color of the crystals found thus far is -bluish, greenish, pinkish brown, yellowish, and colorless. Some very -tiny colorless beryl crystals have been found that are transparent, but -thus far such crystals have been too small to be cut into gems. - -Fine blue beryl crystals have been found in the Franklin Mountains near -El Paso, Texas. Unfortunately, these crystals are so badly flawed and -fractured that they are not suitable for lapidary use. - -It seems likely that careful prospecting of Texas pegmatites will reveal -at least some gem-quality beryl. - - - - - Celestite - - - _Composition_: SrSO_4. _Crystal system_: orthorhombic. _Hardness_: 3.0 - to 3.5. _Specific gravity_: 3.95 to 3.98. _Luster_: vitreous. _Color_: - white, blue, greenish, reddish, and brownish. _Streak_: white. - _Cleavage_: three directions, although one of these directions is not - easily developed. _Fracture_: uneven. _Tenacity_: brittle. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: 1.62 - to 1.63. _Dispersion_: moderate. - -Celestite is very seldom cut into gems. Being very soft, brittle, and -having three cleavages, celestite is completely unsuitable for jewelry. -These same properties make this mineral exceedingly difficult to facet; -however, faceted stones are seen in large collections. - -[Illustration: Fig. 14. Common crystal form of Travis County celestite. - Same crystal form as shown in Plate I, A.] - -Fine crystals of colorless and blue gem-quality celestite (Pl. I, A, and -fig. 14) have been found at Mount Bonnell and other localities west of -Austin, Travis County. The celestite crystals occur in vugs or geodes in -limestone. The crystals are mostly white or colorless and fractured near -the base or where attached, but the tips of the crystals are commonly -clear celestine blue and completely free of flaws. - -Crystals several inches in length have been found, but the average size -is about 1 inch. The smaller crystals are frequently more transparent -and consequently better suited for cutting. It is very difficult to -obtain crystals that will allow the cutting of flawless stones of more -than 4 or 5 carats. - -Bluish and colorless celestite of gem quality and fine crystals have -been found near Lampasas, Lampasas County, and near Brownwood, Brown -County, but neither of these localities has been very productive of good -gem material. - -Celestite geodes have been found in parts of Coke, Fisher, and Nolan -counties, but these geodes contain little gem material. - - - - - Diamond - - - _Composition_: carbon. _Crystal system_: isometric. _Hardness_: 10. - _Specific gravity_: 3.51 to 3.53. _Luster_: adamantine to greasy. - _Color_: brown, colorless, pink, blue, yellow, and various other light - colors; rarely deeply colored; sometimes black. _Cleavage_: four - directions, octahedral, perfect. _Fracture_: conchoidal. _Tenacity_: - brittle. _Diaphaneity_: transparent to opaque. _Refractive index_: - 2.42. _Dispersion_: high. - -There is only one well-authenticated find of diamond in Texas. A small -brownish diamond was found in 1911 on section 64, block 44, Foard County -(Sterrett, 1912, pp. 1040-1041). The exact weight of the stone has not -been recorded, but one authority estimated that it was of sufficient -size and clarity to yield a cut stone of about one-quarter carat. - -The only diamond-bearing rocks known in the United States are in Pike -County, Arkansas. Although many other diamonds have been found in the -United States, all were loose in gravels or streams except for some -stones at the Arkansas locality. The fact that one diamond was found in -Foard County does not mean that the prospects of finding more diamonds -in Texas are much better there than anywhere else in the State. It is -highly unlikely that more than a very few diamonds will ever be found in -Texas, and any stones that may be found in the future are likely to be -widely scattered. - - - - - Epidote - - - _Composition_: HCa_2(Al, Fe)_2Si_3O_1_3. _Crystal system_: monoclinic. - _Hardness_: 6 to 7. _Specific gravity_: 3.25 to 3.5. _Luster_: - vitreous. _Color_: yellowish green to brownish green and brown. - _Streak_: uncolored to grayish. _Cleavage_: two directions. - _Fracture_: uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to - opaque. _Refractive index_: about 1.72 to 1.77. - -Llano County has furnished some green and brownish-green epidote that is -suitable for cutting into cabochons. Most of the material that -approaches gem quality has come from contact metamorphic zones and is -associated with garnet, quartz, and some scheelite. Some small cavities -in the rocks contain tiny transparent crystals of gem quality, but the -largest obtainable flawless faceted stones would probably be less than -15 points. - -Faceted stones of epidote are sometimes known as pistacite owing to -their common pistachio-green color. - - - - - Fluorite - - - _Composition_: CaF_2. _Crystal system_: isometric. _Hardness_: 4. - _Specific gravity_: 3.0 to 3.25. _Luster_: vitreous. _Color_: violet, - blue, colorless, green, yellow, brown, rose, and crimson red. - _Streak_: white. _Cleavage_: four directions, octahedral, perfect. - _Fracture_: subconchoidal to splintery. _Tenacity_: brittle. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: - 1.434. - -Very fine green, transparent fluorite has been found near Voca, Mason -County. The fluorite occurs as vug fillings in pegmatites, associated -with crystals of pink microcline and colorless quartz. Most of the vugs -have been completely filled by the fluorite; therefore, crystals (fig. -15) of the fluorite are not too common. Masses of fluorite several -pounds in weight, rich green, and quite transparent have been found near -Voca. Transparent pieces an inch or more in diameter are common. - - [Illustration: Fig. 15. Common crystal form of fluorite.] - -Fluorite is much too soft for everyday use in jewelry and because of the -low refractive index does not yield brilliant faceted stones. The -perfect four-directional cleavage, relative softness, and brittle -tenacity of the mineral make it difficult to facet. Faceted stones are -seldom seen outside of collections. Cabochons are also difficult to cut -from this material, but the rich color obtained is ample reward for the -time and care necessary in cutting. - -Fluorite occurs at several other localities in Texas, notably in -Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not -commonly in gem quality or colors that warrant its use as gem material. - - - - - Fossil Wood - - -Wood that is buried in silica-rich sediments is commonly replaced by -quartz, agate, or opal. The wood structure, including a large number of -the annular rings, knots, small branches, and bark, may be preserved. -This process of replacement by silica is believed to take considerable -time. Preservations by other means (_see_ Jet, p. 22) are known, but -silica replacements are most commonly used as gem materials. - -Fossil wood is often used by lapidaries as gem material when mineral -replacement preserves the wood structure sufficiently well and when -various impurities color the replacement material attractively. - -Excellent gem-quality fossil wood (Pl. I, B) has been found at a great -number of localities in Texas. Agatized and opalized wood occurs in -great abundance along the outcrops of Eocene and Oligocene strata of the -Texas Gulf Coastal Plain. Much of this material is very well suited for -cabochons, bookends, and other lapidary uses. The preservation is -especially good at numerous localities in Washington, Lee, Fayette, and -Gonzales counties, and the variety of colors, such as bluish, gray, -brown, red, yellow, and black, makes this material especially sought -after by "rock-hounds." Some of the agatized and opalized wood -fluoresces yellow or green under ultra-violet light. The fossil wood is -sometimes found as stumps, limb sections, or large trunk fragments, but -the great majority of the gem material is found as small broken -fragments or stream-rolled cobbles. - -Fossil palm wood is by far the most sought after variety because this -material displays "eyes" and tube-like structures that yield very -attractive cabochons and cabinet specimens. Texas fossil palm wood is -highly regarded by cutters from all parts of the country, and this -material is thought by many lapidaries to be some of the finest -gem-quality fossil wood in the United States. - -Gravel pits and river gravels in Live Oak County have produced very fine -agatized wood. Although the gem material does not seem to be as abundant -in this area as it is in counties to the northeast, the vivid colors and -excellent preservation of the fossil wood in Live Oak County have -attracted collectors from all over the State. The fossil wood usually -occurs as large rounded cobbles in the streams. Much of this material is -quite translucent when cut and contains various shades of brown, orange, -and red. - -The gravels of the Rio Grande have produced some fossil wood in addition -to the excellent agate that is also found there. Most of the fossil wood -found in these gravels is very well preserved, but the colors are -commonly dull shades of brown. Occasional fine red and yellow specimens -have been recovered from the Rio Grande gravels, but these are rare. - -Good agatized wood has been found in and near Palo Duro Canyon, -Armstrong County, about 50 miles southeast of Amarillo. Large trunk -sections are not uncommon, but most of the material of cutting quality -is obtained from small fragments. The Palo Duro Canyon fossil wood -greatly resembles the famous Arizona Petrified Forest wood but is not -nearly as plentiful. The Palo Duro wood contains yellow, brown, red, and -bluish colors most commonly. Some of the wood-producing area is within -Palo Duro Canyon State Park which is, of course, closed to collecting. -The surrounding area has been worked diligently by local collectors, but -new pieces of wood are exposed after heavy rains. - -Webb and Duval counties have also produced some good fossil wood -specimens. - - - - - Gadolinite - - - _Composition_: Be_2FeY_2Si_2O_1_0. (Various other rare-earth elements - may substitute into this mineral structure.) _Crystal system_: - monoclinic. _Hardness_: 6.5 to 7.0. _Specific gravity_: about 4.2. - _Luster_: vitreous to greasy. _Color_: black; in thin splinters dark - bottle green. _Streak_: white to greenish. _Cleavage_: none. - _Fracture_: conchoidal to splintery. _Tenacity_: brittle. - _Diaphaneity_: opaque to subtransparent in thin pieces. _Refractive - index_: variable, about 1.77 to 1.82. - -Gadolinite as a cut gem is not seen outside of large collections; -however, it can be faceted into black opaque stones of little beauty but -of great interest to collectors. The best known locality of this mineral -in the United States is Baringer Hill, Llano County, Texas. -Unfortunately, this locality was completely flooded by the completion of -Buchanan Dam in 1938. Masses and rough crystals of gadolinite weighing -over 100 pounds were mined from this locality. The gadolinite occurred -in a large, very coarse-grained pegmatite dike associated with quartz, -microcline, and fluorite, as well as allanite, fergusonite, nivenite, -cyrtolite, thorogummite, and various other rare minerals. Some of the -minerals in the dike occurred in very large masses. One quartz mass over -40 feet in diameter was noted, and microcline masses up to 30 feet in -diameter were not uncommon. Much of the gadolinite was used by -industrial firms as a source of thorium compounds, although some -specimen and gem material found its way into museums and private -collections. Because the locality was worked mostly from 1910 to about -1925 and because since 1938 the waters of Lake Buchanan have completely -flooded the entire area, material from this locality is now exceedingly -difficult to obtain. The collection of the Smithsonian Institution, -Washington, D.C., contains a cut and polished gem of Baringer Hill -gadolinite that weighs 8.6 carats. This mineral is radioactive because -of the presence of uranium, thorium, and other rare radioactive -elements. - - - - - Garnet - - -The garnet group of minerals is variable in composition. Listed below -are the pure members of this group, but garnets found in nature are -usually a mixture of two or more of these end members. - - Aluminum garnet-- - Grossularite (calcium-aluminum garnet), Ca_3Al_2(SiO_4)_3 - Pyrope (magnesium-aluminum garnet), Mg_3Al_2(SiO_4)_3 - Almandite (iron-aluminum garnet), Fe_3Al_2(SiO_4)_3 - Spessartite (manganese-aluminum garnet), Mn_3Al_2(SiO_4)_3 - Iron garnet-- - Andradite (calcium-iron garnet), Ca_3Fe_2(SiO_4)_3; may contain - magnesium, titanium, and yttrium - Chromium garnet-- - Uvarovite (calcium-chromium garnet), Ca_3Cr_2(SiO_4)_3 - -Since almandite is the only variety of garnet known to occur commonly in -gem quality in Texas, the following properties are for almandite except -where noted. - - _Crystal system_: isometric (all varieties). _Hardness_: about 7.5. - _Specific gravity_: 4.25. _Luster_: vitreous to resinous. _Color_: - red, deep red, and brownish red (other varieties also yellow, white, - orange, pink, black, and green). _Streak_: white. _Cleavage_: none. - _Fracture_: subconchoidal to uneven. _Tenacity_: brittle to tough. - _Diaphaneity_: transparent to subtranslucent. _Refractive index_: - about 1.83. - -Good crystals of gem-quality almandite garnet have been found in Llano, -Blanco, Burnet, and Gillespie counties. In southeast Llano County, -northwest Blanco County, and northeast Gillespie County, the stones -mostly occur in stream gravels where they have collected after being -weathered out of compact mica schists. Owing to the fact that most of -the garnets have not been transported very far from their source, the -stones commonly show good crystal form (Pl. II, A). All of the garnets -from one locality commonly do not have exactly the same crystal form. -The garnets are mostly widely scattered in the stream gravels but can be -found concentrated behind rocks and on small gravel bars. - -Many of the crystals are less than one-eighth inch in diameter; however, -good crystals one-fourth to one-half inch in diameter are common. Most -of the stones are too fractured or have too many inclusions to yield -gems, but many transparent stones have been found. The transparent -crystals usually yield flawless deep red faceted stones of 2 carats or -less. Some of the stones that contain too many inclusions to facet are -cut as cabochons and are then often known as carbuncle. - -Small garnet fragments have been found in streams and in gneisses and -pegmatites near Castell, Llano County, but they are not commonly of gem -quality. - -Occasional small gem-quality garnets have been found in pegmatites and -contact metamorphic zones in Burnet County. Garnets have also been found -in several other counties, notably Mason, El Paso, Hudspeth, and -Culberson, but no stones of facet quality have been reported. - - - - - Jet - - - _Composition_: a variety of brown coal or lignite. _Structure_: woody. - _Hardness_: 3 to 4. _Specific gravity_: about 1.30 to 1.35. _Luster_: - dull. _Color_: black, brownish black. _Streak_: brown to brownish - black. _Cleavage_: none. _Fracture_: uneven to smooth. _Tenacity_: - tough to slightly brittle. _Diaphaneity_: opaque. Burns with a sooty - yellowish flame. - -Jet is a type of fossil wood in which there has been sufficient chemical -change to make the wood relatively hard and black without destroying the -woody structure. The best specimens of jet polish into lustrous black -cabochons. - -Jet occurs in Presidio County as compressed and flattened trunks of -trees in a thin layer of coal and lignite in Cretaceous strata 100 to -200 feet stratigraphically below the San Carlos beds. - -Specimens of "jet" have been found in some of the lignitic Tertiary -strata of the Texas Gulf Coastal Plain; however, this material is mostly -soft, brownish, and not of gem quality. - - - - - Labradorite - - - _Composition_: NaAlSi_3O_8, 50% to 30%; CaAl_2Si_2O_8, 50% to 70%. - _Crystal system_: triclinic. _Hardness_: 6.0 to 6.5. _Specific - gravity_: about 2.60. _Luster_: vitreous to sometimes pearly. _Color_: - straw yellow, white, greenish, gray, reddish, bluish, and green. - Sometimes shows a play of colors on particular cleavage surfaces. - _Streak_: uncolored. _Cleavage_: three directions. _Fracture_: uneven - to conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to - translucent. _Refractive index_: about 1.56. _Dispersion_: low. - -Very fine facet-quality labradorite has been found about 20 miles south -of Alpine, Brewster County. The labradorite occurs loose in the soil as -slightly weathered or frosted cleavage fragments, commonly showing one -or more crystal faces (Pl. II, B). The pale-yellow or straw-yellow color -of these fragments, as well as their lack of internal imperfections, -makes these stones excellent gem material. Individual pieces that exceed -three-fourths inch in their longest dimensions are rare. Cut stones of -more than 5 or 6 carats from this locality are scarce. The source of -this material is uncertain, but it is probably weathering out of an -underlying igneous rock. - - - - - Microcline - - - _Composition_: KAlSi_3O_8. _Crystal system_: triclinic. _Hardness_: - 6.0 to 6.5. _Specific gravity_: 2.54 to 2.57. _Luster_: vitreous to - pearly. _Color_: white, pale yellow, red, blue green, bluish. - _Streak_: white. _Cleavage_: four directions, usually three of these - distinct. _Fracture_: uneven. _Tenacity_: brittle _Diaphaneity_: - transparent to translucent. _Refractive index_: about 1.52 to 1.53. - -Very fine crystals of blue microcline have been found east of Packsaddle -Mountain and near Kingsland in Llano County. Crystals exceeding 1 foot -in length have been found, although most are only a few inches long. The -color of the microcline is mostly pale blue, but some crystals are -darker. Microcline crystals associated with milky or vein quartz, smoky -quartz, some biotite, and rarely cassiterite occur in pegmatite dikes -which vary in size from a few inches to several feet in thickness. The -color of this microcline is pale in comparison to microcline from some -other localities in the United States, but the Texas blue microcline -does yield pleasing cabochons. Perfect crystals of this material are -prized by collectors. Blue or greenish microcline is often called -amazonite or amazon stone. - -Bluish microcline associated with quartz and topaz has also been -reported near Katemcy, Mason County. - -Red microcline is common in several central Texas counties and is a -primary constituent of many of the igneous rocks in those counties. -Large crystals of perthitic red microcline occur in pegmatite dikes of -Mason, Llano, Burnet, and Gillespie counties. Any feldspar quarry or -other pegmatite mining operation in any of these counties is likely to -contain large red microcline crystals and fragments. Unfortunately, the -good crystals that may have been present are often shattered by blasting -during quarrying operations. - -Feldspar quarries in northeastern Gillespie County have yielded some -good red cabochon material as well as good crystals. Here the microcline -occurs with milky and smoky vein quartz, smoky quartz crystals, clear -quartz crystals, greenish muscovite, and biotite. Many of the older -quarries in Gillespie County have not been active for some time, and the -dumps and quarry walls have been diligently searched by collectors. - - [Illustration: Fig. 16. Crystal faces on microcline specimen shown in - Plate III, A.] - -Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet -counties have produced some good red microcline specimens and cutting -material (Pl. III, A, and fig. 16). Many of these crystals are more -pinkish than those in Gillespie County, but this is commonly due to the -fact that the crystal faces of the Lake Buchanan area crystals are -somewhat more weathered than the fresh Gillespie County crystals. - -Numerous other local areas in the counties mentioned, as well as some -localities in Hudspeth and Culberson counties, have also produced small -amounts of red and pink microcline of gem quality. - - - - - Obsidian - - - _Composition_: volcanic glass. _Structure_: amorphous. _Hardness_: 5.0 - to 5.5. _Specific gravity_: 2.3 to 2.5. _Luster_: vitreous. _Color_: - black, dark gray, reddish, brown, bluish, and greenish. _Streak_: - white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: translucent to nearly opaque. _Refractive index_: - variable, about 1.45 to 1.53. - -Gem-quality black and dark-gray obsidian has been found in Presidio -County associated with extrusive igneous rocks. The obsidian in this -area is too opaque to serve as attractive faceted stones but is found in -pieces of sufficient size and quality to yield nice cabochons. Some of -the small weathered pieces of this material resemble tektite in outward -appearance; in fact, the "valverdites" mistaken originally for tektites -are pebbles of weathered obsidian in terrace gravel of Val Verde County. -Obsidian takes a high polish but is very sensitive to heat. Stones that -are slightly overheated during grinding or sanding will quickly shatter. - -Obsidian of gem quality has been reported also in Brewster County. - - - - - Opal - - - _Composition_: SiO_2nH_2O. _Structure_: amorphous. _Hardness_: 5.5 to - 6.5. _Specific gravity_: 1.9 to 2.3. _Luster_: subvitreous to pearly. - _Color_: white, bluish, pink, brown, yellow, and gray. _Streak_: - white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: transparent to nearly opaque. _Refractive index_: 1.43. - -Opal other than as fossil or opalized wood (pp. 20-21) occurs at the -following several localities in Texas. - -Approximately 16 miles south of Alpine, Brewster County, precious opal -occurs in very small seams and as cavity fillings in very hard -pinkish-brown rhyolite. This opal is milky or bluish and commonly -exhibits small flashes of blue, green, red, and orange fire. Individual -pieces of this opal are mostly quite small, rarely over one-fourth inch -in diameter, and very difficult to remove from the tough rhyolite -matrix. Local lapidaries have cut interesting cabochons from this -material in which several small patches of opal that are close together -in the matrix are included in the same cabochon. - -Small finds of opal associated with rhyolites and basalts have come from -other localities in west Texas, but the opal mostly does not display -enough play of colors to warrant its use as gem material. - -Near Freer, Duval County, some very attractive common opal has been -found. The opal is colored various shades of pink, blue, and yellow and -in certain local areas occurs as fragments that are cemented together by -clear chalcedony. Various colors are commonly found in the same piece, -and such material yields handsome cabochons. Although the area has never -been worked commercially, it has been hunted by collectors and cutters -for several years. - - - - - Pearl - - -Pearls are the result of the secretion of calcium carbonate by various -shellfish around sand grains, parasitic organisms, shell fragments, or -other foreign objects that have in some way entered the body cavity of -the shellfish. Since the shellfish is unable to expel these irritating -particles or organisms, it deposits successive layers of calcium -carbonate around the foreign substance to make it smoother and less -irritating. Although pearls are principally calcium carbonate, they also -contain small amounts of an organic substance, called conchiolin, and -water. Pearls are found in shellfish that live in either fresh or salt -water. Few pearls are spherical in shape; most are rounded but somewhat -irregular and are known as baroque pearls. Good quality pearls are the -only gemstone commonly sold by the grain, a unit of weight equal to 0.25 -carat or 0.05 gram. The pearl grain is not the same unit of weight as -the Troy grain. - -In Texas, pearls have been found in fresh-water clams in most of the -major rivers and streams, notably in the Brazos, Concho, Colorado, -Guadalupe, Llano, Nueces, Sabine, Rio Grande, and Trinity Rivers. -Several Texas lakes have also yielded pearls, notably Caddo Lake and -other lakes in north-central and northeast Texas. - -Small pearls are frequently found along the Texas Gulf Coast in edible -oysters and other common shellfish. Fossil pearls have also been found -but because of their darkened appearance are of value only as -curiosities. - -The pearls thus far found in Texas have been of relatively poor quality -and show little or no iridescence. These pearls have little value except -as curiosities, although one writer has stated that the discovery of -pearls in the Nueces River led to the original Spanish settlement of the -State (Baker, 1935, p. 569). - - - - - Quartz - - - _Composition_: SiO_2. _Crystal system_: hexagonal. _Hardness_: 7. - _Specific gravity_: 2.65 to 2.66 in crystals. _Luster_: vitreous, also - waxy, greasy, and dull. _Color_: most often colorless, brown, yellow, - violet; sometimes green, red, blue, and black; cryptocrystalline - varieties often variously colored by impurities. _Streak_: white. - _Cleavage_: indistinct. _Fracture_: conchoidal to splintery. - _Tenacity_: brittle to tough. _Diaphaneity_: transparent to opaque. - _Refractive index_: 1.544 to 1.553. - -The quartz family gemstones can be divided into two groups for purposes -of description. The first group is the crystalline varieties, or those -quartz varieties that commonly occur in distinct crystals. The second -group is the cryptocrystalline varieties, or those quartz varieties that -occur as irregular masses that are composed of many microscopic -crystals. The crystalline varieties are usually much more transparent -and are most often seen as faceted stones. The cryptocrystalline -varieties vary from subtransparent to opaque and are almost always cut -as cabochons. - - - - - CRYSTALLINE VARIETIES - - -_Amethyst_ (violet to purple-colored quartz).--A northeastern Gillespie -County locality known as Amethyst Hill has produced quite a number of -fine light to medium violet amethyst crystals which occur in quartz -veins and geodes associated with serpentine and talc. Many crystals have -been found loose in the soil. - -The amethyst tends to be very irregularly colored in zones parallel to -the crystal faces. In many, the base of the crystal is colorless or -white and only the termination is violet. Crystals up to 3 inches long -have been found at this locality, but the average size is much less. - -The surface at this locality is almost entirely depleted of amethyst, -with only an occasional small crystal or fragment to be seen. However, -small excavations are still sometimes productive. - -Good groups of pale amethyst crystals have been found in quartz veins -near the old town site of Oxford, Llano County. The occurrence seems to -be much the same as the Amethyst Hill locality. Little exploration for -gemstones has been done in this area, and future discoveries seem -likely. - -Chalcedony geodes lined with amethyst crystals have been found in -Brewster, Presidio, Culberson, and Hudspeth counties, but the -occurrences are scattered. The crystals are seldom large enough to yield -gems of more than 3 carats and are mostly very light colored. - -A few pieces of gem-quality amethyst have been found in Burnet County. - -_Citrine_ (yellow quartz).--Very little gem-quality citrine has been -reported in Texas. Some small citrine crystals have been found at -Amethyst Hill in northeastern Gillespie County, but few are of -sufficient size or color to yield good gems. - -The writer has seen one citrine crystal that was found in the gravels of -a small stream in eastern Llano County near Buchanan Dam. The crystal -weighs about 1 ounce and is perfectly clear, light golden yellow, and -flawless. However, a further search of the stream gravels failed to -produce any other citrines. - -_Rock crystal_ (colorless quartz).--Numerous localities in Texas produce -this colorless variety of quartz, which is the most common variety of -facet quality quartz and consequently is of little value. - -Rock crystal occurs at many localities in Burnet, Llano, and Mason -counties. The crystals mostly occur in pegmatite dikes or in stream -gravels where they have been weathered out of their parent rock. Some -fine colorless quartz crystals have been found near Voca, Mason County, -in weathered pegmatite dikes and also loose in the sands of nearby -streams. Crystals from this locality are often stained with reddish iron -oxide on their outer surfaces. Some of the rock crystal found near -Katemcy, Mason County, shows asterism when cut with the proper -orientation. Fine clear colorless crystals up to 8 inches long have been -found in the pegmatite dikes near Lake Buchanan in both Llano and Burnet -counties. Several localities near Enchanted Rock in Llano County have -also produced some good colorless crystals. - -Feldspar quarries in large pegmatites in northeastern Gillespie County -have yielded attractive quartz crystals, some of which contain smoky -phantom crystals and tourmaline inclusions. - -Some pieces of rock crystal enclosing green, needle-like actinolite -crystals have been found near the Llano-Gillespie-Blanco County corner. -This material is not suitable for faceted gems but does lend itself to -interesting and attractive cabochons. - -Colorless quartz crystals commonly are found lining small chalcedony -geodes in Brewster, Presidio, Culberson, Hudspeth, Reeves, and Jeff -Davis counties. These crystals are most commonly less than 1 inch long -but are mostly very clear. - -Rock crystal has been found in crevices of petrified wood in many east -and southeast Texas counties, although the crystals are mostly quite -small. - -Many lesser occurrences of rock crystal, too numerous to mention, are -located within the State. - -_Rose quartz_ (pink quartz).--Rose quartz occurs at various localities -in Burnet, Llano, Mason, and Gillespie counties, but the amount of -material is mostly small and the greater part unsuitable for gem -purposes. Some good pink rose quartz occurs near Town Mountain, Llano -County, but this material does not have flawless areas large enough to -yield faceted stones of more than a few carats. Rose quartz is always -slightly milky, or cloudy, and does not cut into brilliant faceted -stones. The Town Mountain rose quartz has been cut into attractive -cabochons. - -_Smoky quartz_ (brown, yellow-brown, and golden-brown quartz).--Several -Texas localities have produced fine smoky quartz. Baringer Hill, a noted -rare-earth minerals pegmatite locality in Llano County, contained some -smoky quartz crystals that were estimated to weigh over 1,000 pounds, -and the locality produced many smaller crystals that were of gem -quality. Baringer Hill was flooded by the completion of Buchanan Dam in -1938 and is presently under the waters of Lake Buchanan. A few fine -golden-brown gem-quality crystals have been found along the lake shore -and in small pegmatites nearby (Pl. III, B.). - -Feldspar quarries in northeastern Gillespie County have produced smoky -quartz crystals that exceed 1 foot in length, but these crystals are -mostly flawed, possibly as a result of blasting, and mostly contain only -small clear areas. - -Good color smoky quartz crystals are found with topaz in the pegmatites -and stream beds in Mason County, near Streeter, Grit, and Katemcy. These -crystals tend to be lighter colored than those near Lake Buchanan, but -they commonly contain large flawless areas. - - - - - CRYPTOCRYSTALLINE VARIETIES - - -_Chalcedony._--When free from impurities of various oxides and other -compounds, chalcedony has little to render it pleasing as a gemstone. It -is mostly gray, white, brown, or bluish and commonly has a waxy luster. -Some of the chalcedony found along the Rio Grande Valley and in west -Texas will take dyes, and local lapidaries have had some success in -dyeing this material various shades of blue, green, yellow, and red. -When the chalcedony is naturally colored and variegated, usually in -bands, mossy figures, or dendritic forms, it is called agate. - -_Agate_ (variegated chalcedony).--The wide variety of markings and -colors available together with the ease of cutting make agate a favorite -of many lapidaries. Fine agate has been found at numerous localities in -west and south Texas. Fine plume agate, famous throughout the United -States, is found south of Alpine. Plume agate is characterized by -dendritic or tree-like inclusions and is mostly cut into very handsome -cabochons. The agate from south of Alpine commonly contains black, red, -yellow, or brown plumes within the same piece. The variety of colors and -lack of porosity of this agate make it highly desired among lapidaries. -The agate occurs loose on the surface of the ground and in the soil in -small nodules that have a very rough, brownish surface. These nodules -are mostly less than 3 inches in diameter, although specimens of gem -quality have been found that exceed 200 pounds. - -Some very fine agate has been found in the vicinity of Needle Peak, -Presidio County. This material is mostly green moss agate in clear -chalcedony and commonly contains small yellow "sun-burst" figures. The -contrasting yellow and green design makes very beautiful cabochons. - -Fine agate has been found south of Marfa, Presidio County. This agate is -mostly clear chalcedony with black, yellow, or variously colored plumes, -moss, or "bouquet-like" figures. - -Numerous other localities in Presidio and Brewster counties have -produced good agate. - -Various amounts of agate, jasper, and chalcedony occur in the gravels of -the Rio Grande in varying quantities from Big Bend National Park -downstream to Brownsville. This agate is found both in the present river -gravels and in the older river gravels that now are located on nearby -hills and slopes up to several miles north or south of the present Rio -Grande. The greatest concentration of agate and related gem materials -seems to be in the area between Laredo and Rio Grande City. Vast -quantities of excellent gem material have been removed from this area -for many years (Pl. IV). The agate occurs as rounded, stream-worn -cobbles and commonly has a thin white coating that makes it difficult to -distinguish from the abundant chert and other rocks. The agate occurs in -cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem -quality that exceed twice this size are known. The agate varies greatly -in design and color. Plume, moss, banded, and sagenitic agate occur in -these gravels in a wide variety of colors. The jasper in the Rio Grande -gravels is yellow, red, green, or various shades of these and is -commonly suspended as angular fragments in clear chalcedony. - -Good agate has also been found near Balmorhea in Reeves and Jeff Davis -counties and in smaller amounts at numerous other west and south Texas -localities. - -_Agatized wood_ (_see_ Fossil wood, pp. 20-21). - -_Carnelian_ (translucent reddish chalcedony).--This variety of -chalcedony in small quantities has been reported from near Van Horn, -Hudspeth County. Small pieces of carnelian have been found in the -gravels of the Rio Grande, but finds have been few and scattered. - -_Jasper_ (impure opaque or subtranslucent quartz).--Good green, yellow, -red, and brown jasper has been found in the gravels of the Rio Grande at -all of the localities that produce agate. The colors are quite vivid, -and the material takes a fine polish. Some pieces of orbicular jasper -(jasper with circular or eye-like markings) have been found in this -material. These gravels commonly contain jasper as fragments that are -suspended in clear chalcedony; this is called brecciated jasper and -yields very handsome cabochons. - -Many of the west Texas agate localities also produce jasper in quantity. -Good jasper has been reported from north of Brackettville, Kinney -County. Jasper is a minor constituent of the stream gravels in many -parts of the State. - - - - - Sanidine - - - _Composition_: KAlSi_3O_8; commonly contains some sodium. _Crystal - system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58. - _Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow, - and gray. _Streak_: uncolored. _Cleavage_: three directions. - _Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: - transparent to subtranslucent. _Refractive index_: 1.52 to 1.53. - -Some feldspars, including sanidine, show a nice blue sheen in reflected -light parallel to certain crystallographic directions. Stones having -this property are called moonstone. A clear yellowish sanidine showing -an attractive blue sheen has been found in Brewster, Jeff Davis, and -Presidio counties. The individual pieces are small, the average size -being about one-eighth inch. The sanidine is found loose in the soil at -some localities where it has weathered out of rhyolite, and specimens of -the sanidine in the parent rock are not difficult to obtain. Very small -cabochons can be cut from this material, but few lapidaries have done so -because inexpensive larger pieces of moonstone can be obtained easily -from foreign sources. However, the west Texas sanidine does show a blue -sheen when cut and polished. - - - - - Spinel - - - _Composition_: MgAl_2O_4 (magnesium may be replaced in part by ferrous - iron or manganese and the aluminum by ferric iron and chromium). - _Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to - 4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red, - blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_: - one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle. - _Diaphaneity_: transparent to opaque. _Refractive index_: variable, - approximately 1.72 to 2.00. - -In many areas of the world, fine quality, beautifully colored, -transparent spinels are found and used as gems. The only gem-quality -spinel reported thus far in Texas is black and opaque. Near Eagle Flat -in Hudspeth County, black spinel crystals have been found associated -with augite and natural glass; these minerals are weathering out of an -intrusive igneous rock. The spinel crystals have an octahedral form -which is common for this mineral (fig. 17). Most of the spinels are free -of flaws, but because of their black color they have little value as -gems. The crystals are found loose in the sand of streams near the -outcrops of the igneous rock or embedded in the rock. They seldom exceed -half an inch in diameter. These stones are primarily sought by -collectors. - - [Illustration: Fig. 17. Common crystal form of spinel.] - - - - - Tektite (Bediasite) - - - _Composition_: A natural glass, approximately 75% SiO_2, 15% Al_2O_3, - 4% FeO, also MgO, Na_2O, K_2O, and traces of other elements. _Crystal - structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to - 2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_: - dark brown, greenish brown, appears black in thick sections. _Streak_: - uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: - brittle. _Diaphaneity_: transparent to subtransparent. _Refractive - index_: 1.488 to 1.512. - -The average bediasite size is about 1 inch in diameter, although -specimens approximately 3 inches in diameter are known. The uncut -tektites are very interesting, showing a variety of shapes and surface -features (Pl. V, A) and many exhibit contorted flow structure. The -surface of many tektites is grooved or furrowed, while on others it is -smooth or frosted. The Texas tektites are known as "bediasites," after -place names in Grimes County traceable to the Bedias Indians who -formerly lived there. - -Dark brown and greenish-brown tektites have been found in Texas in -gravels at scattered localities in Walker, Grimes, Brazos, Burleson, -Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas -the only other authenticated tektite localities in the United States at -the present time are in Dodge and Irwin counties, Georgia. A fragment of -a similar tektite has recently been reported from near Martha's -Vineyard, Massachusetts. The tektites reported from Oklahoma are now -known to be pebbles of obsidian. - -Although tektites have little value or beauty as gemstones, they have -been cut by lapidaries as both faceted and cabochon stones. Tektites -take a high polish but are mostly so dark in color that they appear -black. - -The origin of tektites is of great scientific interest and is currently -the subject of much debate. Some scientists believe that tektites are of -meteoritic origin, while others believe that tektites were formed by -various terrestrial processes. Since no one has actually observed a -tektite to fall or form, and many of the theories of origin are -difficult to prove without direct observation, the origin of tektites is -likely to remain in controversy for some time. - - - - - Topaz - - - _Composition_: Al_2(F, OH)_2SiO_4. _Crystal system_: orthorhombic. - _Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous. - _Color_: pale blue, sky blue, greenish, white, wine yellow, straw - yellow, grayish, pink, reddish, and orange. _Streak_: uncolored. - _Cleavage_: one direction, basal, highly perfect. _Fracture_: - conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent - to subtranslucent. _Refractive index_: about 1.60 to 1.63. - _Dispersion_: moderate. - -Various yellow and smoky colored quartz gems are offered for sale as -"Spanish Topaz," "Smoky Topaz," "Madeira Topaz," and "Topaz Quartz." -These names are entirely misleading and should be dropped from usage. - -Fine gem-quality white, pale-blue, and sky-blue topaz has been found -near Streeter, Grit, and Katemcy, Mason County. This Texas gem material -compares favorably in color, size, and clarity with topaz found anywhere -in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18) -occasionally are found in pegmatite dikes associated with quartz, black -tourmaline, cassiterite, and pink microcline. Many of the gem-bearing -pegmatites have been eroded away, leaving the topaz concentrated in the -stream beds. The stones mostly occur as frosted, stream-worn pebbles -(Pl. VI, A) in the numerous small creeks in the area. The topaz is -heavier than the quartz and microcline that compose the stream gravel -and is commonly found immediately on top of the granite bed-rock in the -bottom of the stream bed. The stones tend to lodge behind boulders or -small dikes cutting across the stream. - - [Illustration: Fig. 18. Crystal faces on topaz crystal shown in Plate - V, B. This crystal habit is typical of the topaz from Mason County.] - -The white or colorless stones are by far the most common, outnumbering -the bluish stones about ten to one. The color of the blue stones tends -to be irregularly distributed in zones parallel to the crystal faces. -Topaz that is colored in this manner should be cut with the best blue -color near the bottom or culet of the gem (fig. 19). If done correctly, -this will give the entire gemstone the desirable blue color. - -[Illustration: Fig. 19. Cross section showing the proper orientation of - dark-color zone in a gem cut from an irregularly colored stone.] - - COLORLESS - BLUE - -The colorless stones can be turned pale yellow, yellowish brown, or -straw yellow by exposure to X-ray radiation, and some of the bluish -stones will fluoresce faintly yellowish under ultra-violet light. - -The largest gem-quality topaz crystal yet found in North America has -come from Mason County. It is a pale-blue crystal weighing 1,296 grams, -now in the collection of the U.S. National Museum. Several other large -pieces, some weighing over a pound, have been found. One large crystal, -exact weight unknown, was found near Katemcy. Several gem cutters have -estimated that this stone could easily yield a single, flawless -pale-blue gem of about 500 carats. Many large gems have been cut from -topaz found in this area, including at least one stone of over 300 -carats. - -One obstacle in the cutting of topaz is its perfect basal cleavage. The -gemstone should be oriented so that no facet of the stone will be -parallel to or within less than about 5 degrees of the cleavage -direction, or the facet may be very difficult or impossible to polish. - -It is difficult to estimate the productivity of this area since its -discovery in the early 1900's. Few systematic attempts have been made to -exploit the deposits, and a great amount of the topaz thus far recovered -has been found by private collectors. The Mason County topaz deposits -are still very productive, and additional exploration may uncover even -more gem-producing areas. - -Topaz has also been found in stream gravels or pegmatites in Burnet, -Llano, Gillespie, and El Paso counties but very rarely in gem quality. - - - - - Tourmaline - - - _Composition_: H_9Al_3(BOH)_2Si_4O_1_9; hydrogen often replaced by - iron, magnesium, calcium, or fluorine. _Crystal system_: hexagonal. - _Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_: - vitreous to resinous. _Color_: black, brownish black, brown, blue, - green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_: - two directions, very imperfect. _Fracture_: subconchoidal to uneven. - _Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive - index_: about 1.62 to 1.64. - -Black tourmaline is schorl; brown tourmaline, dravite. - -Good crystals of black and dark brown tourmaline occur at Town Mountain -near Llano, Llano County. The tourmaline crystals average about 1 inch -in length, do not commonly exceed 2 inches, and are associated with -white vein quartz. The quartz completely encloses the tourmaline, but -the crystals can be broken free or the quartz can be trimmed away with -the use of a diamond saw. The latter procedure is recommended whenever -possible, for it is very easy to shatter the tourmaline crystals while -trying to remove them from the quartz by other means. Many of the -crystals are completely unsuitable for cutting, being too brittle or too -badly cracked and flawed. However, some small crystals have been found -that are of sufficient quality and size to yield flawless stones of a -few carats. Few of these stones have been cut since the tourmaline is so -dark that it appears opaque, and few persons find a gem of this nature -attractive. - -Good black and dark brown crystals of tourmaline associated with -andalusite and graphite occur in the Packsaddle schist (Precambrian) -near Sunrise Beach, Llano County (Pl. VI, B, and fig. 20). Although -generally smaller in diameter than the crystals found at Town Mountain, -they commonly exceed 3 inches in length, although the average size is a -little over 1 inch. Many of these crystals are suitable for cutting into -opaque or nearly opaque stones of about 5 or 6 carats. - -Black tourmaline has also been found in Hudspeth and Culberson counties -but not of sufficient quality to be used as a gemstone. - -[Illustration: Fig. 20. Common crystal form of Llano County tourmaline.] - - - - - Turquoise - - - _Composition_: hydrous phosphate of aluminum and copper. _Crystal - system_: triclinic. _Hardness_: 5 to 6. _Specific gravity_: variable, - 2.6 to about 2.8. _Luster_: dull, sometimes waxy. _Color_: sky blue to - greenish blue. _Streak_: white to greenish. _Cleavage_: none in - massive material, two directions in crystals. _Fracture_: conchoidal - to subconchoidal. _Tenacity_: brittle. _Diaphaneity_: subtranslucent - to opaque. _Refractive index_: 1.61 to 1.65. - -Turquoise of good sky-blue to greenish-blue color has been found a few -miles southwest of Van Horn, Culberson County. Several shallow pits were -dug at this locality about 1910; however, the amount of turquoise -produced was small. The main occurrence of the turquoise was in seams -about 1 millimeter thick along joints in the fine-grained rocks of this -area. Persons who have visited Culberson County more recently report -that even minute traces of the turquoise are now difficult to find at -the old prospect pits. However, further prospecting in the area might -yield some additional localities. - -Small amounts of turquoise have been reported near El Paso, El Paso -County, and also in volcanic rocks near the Jeff Davis-Brewster County -line, north of Alpine. - -A small amount of turquoise has been mined from several localities a few -miles northwest of Sierra Blanca in the Sierra Blanca Mountains of -Hudspeth County. - - - - - GLOSSARY - - - Amorphous--without definite molecular structure; not crystalline. - Baroque stone--an irregularly shaped, polished stone; usually applied - to tumbled stones. - Baroque pearl--an irregularly shaped pearl. - Brilliancy--reflecting much light; having brightness. - Brilliant cut--a mode of arrangement of facets commonly used on round - or oval stones. The standard American brilliant cut has 57 or - 58 facets. Most diamonds of 5 or less carats are cut in this - manner. - Cabochon--a stone cut with a flat or convex upper surface; sometimes - faceted in part. Opal, star sapphire, and agate are stones - that are frequently cut in this style (fig. 2). - Cambrian--a division of geologic time, estimated to be the time from - 550 to 440 million years ago; the oldest time division of the - Paleozoic era. - Carat--a unit of weight equal to 1/5 of a gram or 0.2 gram. One ounce - avoirdupois is equal to 141.75 carats. - Cleavage--the tendency of certain minerals to split in particular - directions yielding relatively smooth plane surfaces. - Conchiolin--an organic albuminoid substance found in pearls. - Conchoidal--a type of fracture having curved concavities or the - approximate shape of one-half of a bivalve shell. Glass has - excellent conchoidal fracture. - Cretaceous--a division of geologic time, estimated to be the time from - 135 to 60 million years ago; youngest division of the Mesozoic - era. - Crown--that portion of a faceted gem above the girdle; the upper - portion of a facet-cut gem (fig. 6). - Cryptocrystalline--composed of very fine or microscopic crystals. - Crystal--the regular polyhedral form, bounded by plane surfaces, that - is assumed by a mineral under suitable conditions. Crystals - have definite external symmetry and internal molecular order. - Crystalline--possessing definite internal molecular order; not - amorphous. - Cubic--in the general shape of a cube. The isometric crystal system is - often called the cubic system. - Culet--the very bottom portion of a faceted gem; the point or line - formed by the intersection of the lowest pavilion facets (fig. - 6). - Dendritic--branching or tree-like in form. - Diaphaneity--relative transparency. The diaphaneity of a mineral is - described as transparent, translucent, opaque, etc. - Dike--a tabular rock body, usually igneous in origin, which cuts - across the surrounding rock strata. - Dispersion--a measure of the ability of gemstones to separate complex - or white light into its component colors; often illustrated - with a prism. Gemstones that are capable of separating colors - of light widely are said to have high dispersion; gemstones - not so capable of separating white light into colors are said - to have low dispersion. - Dopping--the act of cementing a gemstone, either rough or partly - finished, to a dop-stick. - Dop-stick--the wooden stick or cylindrical piece of metal to which a - gemstone is cemented to facilitate handling during cutting and - polishing. - Dop-wax--the agent or cement used to secure a gemstone to a dop-stick. - Emerald cut--a rectangular or square faceted stone with beveled - corners whose surfaces are covered with several series of - rectangular facets. - Eocene--a division of geologic time, estimated to be the time from 50 - to 40 million years ago; one of the older divisions of the - Cenozoic era. - Extrusive rock--igneous rock that has been extruded or forced out onto - the earth's surface. - Facet--a single plane polished surface on a faceted gem. - Facet head--a device used in the cutting and polishing of faceted - gems; used to control the placement of facets and their - relative angles (fig. 7). - Facet table--the equipment used in the cutting and polishing of - faceted gems and the table on which most of the equipment is - mounted (fig. 7). - Feldspar--a group of closely related silicate minerals including - orthoclase, microcline, sanidine, plagioclase, labradorite, - and others. - Fire--the reflections of variously colored light from a precious opal; - also the different colors of light reflected from a faceted - gem owing to the dispersion of the mineral. - Fracture--the texture of a freshly broken surface other than a - cleavage surface, described as conchoidal, even, splintery, - etc. - Gem--a cut and polished gemstone. - Gemology--the science dealing with the study of gemstones. - Gemstone--a mineral suitable for cutting into a gem; the term - gemstones is frequently used collectively to include both cut - and polished stones and rough stones. - Geode--a rounded or spherical rock cavity; commonly lined with - crystals. - Girdle--the portion of a faceted gem separating the crown from the - pavilion; the girdle may or may not be polished and usually - contains about 2 percent of the total depth of the gem (fig. - 6). - Gneiss--a coarse-grained metamorphic rock having segregations of - granular and platy minerals that give it a more or less banded - appearance without well-developed schistosity. - Grain (pearl grain)--a unit of weight equal to 0.05 gram or 0.25 - carat; not the same as the Troy grain. - Granite--a granular igneous rock composed mostly of quartz, feldspar, - and commonly mica and/or hornblende. - Hexagonal--having six angles and six sides; a crystal system in which - the crystal faces are referred to four intersecting axes; - three of these axes are equal, lie in the same plane, and - intersect at angles of 60 degrees; the fourth axis is - perpendicular to the other three. - Igneous rock--rock formed by solidification from a hot melt. - Index of refraction--a measure of the relative ability of a gemstone - to "bend" incident light rays; sine of the angle of incidence - of a light ray divided by the sine of the angle of refraction. - Intrusive rock--rock that has been pushed (usually in a molten state) - among pre-existing rock strata, commonly along faults or - fissures. Intrusive rocks do not reach the earth's surface but - are commonly exposed at the surface by later erosion. - Isometric--a crystal system in which the crystal faces are referred to - three equal intersecting axes at right angles to each other. - Lap--a disc-shaped piece of metal or other material which is - impregnated with diamond dust, or some other cutting or - polishing agent, that is revolved while the gemstone is worked - against it. - Lap plate--a metal plate to which a cutting or polishing lap is - attached, usually by means of a threaded bolt and wing nut. - The lap plate is attached to the shaft which is turned by the - motor under the facet table. - Lapidary--one who practices the lapidary arts; a gem cutter. - Limestone--a sedimentary rock composed mostly of calcium carbonate. - Luster--the appearance of the freshly broken or unweathered surface of - a mineral in reflected light (p. 5). - Main facet--as applied to the standard American brilliant cut, one of - the first eight facets cut on either the crown or pavilion of - a gem (fig. 6). - Matrix--the material in which a specific mineral is embedded; also the - rock to which one end of a crystal is attached. - Metamorphic rock--rock that has been changed from its original state - by heat, pressure, chemical action, or some combination of - these factors. - Millimeter--^1/_1_0 centimeter; approximately ^1/_2_5 inch. - Mineralogy--the science concerned with the study of minerals, - including their occurrence, composition, forms, properties, - and structure. - Monoclinic--a crystal system in which the crystal faces are described - in relation to three intersecting unequal axes, two of which - are at right angles and the third inclined. - Oligocene--a division of geologic time, estimated to be the time from - 40 to 28 million years ago; part of the Cenozoic era. - Opaque--does not transmit light. - Orbicular--containing orbs or spherical or eye-like markings or - structures. - Orthorhombic--a crystal system in which crystal faces are referred to - three unequal intersecting axes at right angles. - Pavilion--the portion of a faceted gem below the girdle (fig. 6). - Pegmatite--a body of coarse-grained intrusive igneous rock, commonly - lens or dike shaped. - Perthitic--a plaid-patterned structure resulting from intermixture of - soda- and potash-rich feldspars. - Phantom crystal--a crystal outline seen within another crystal, mostly - due to entrapping of inclusions during the crystal's growth. - Pleochroism--the property of transmitting different colors of light in - different crystallographic directions. - Point--a unit of weight equal to ^1/_1_0_0 (0.01) carat. - Porous--containing pores or void spaces. - Precambrian--a division of geologic time, estimated to be all of - geologic time prior to 550 million years ago; the time before - the Paleozoic era. - Preform--a gemstone that has been ground to a rough outline of the - finished shape of a gem. - Rhyolite--a fine-grained extrusive or shallow intrusive igneous rock - of approximately the same composition as granite. - Rough--uncut, not worked by a lapidary, not cut and polished. - Schist--a metamorphic rock that contains an abundance of oriented - platy minerals that enable the rock to be split with relative - ease parallel to the flat surfaces of the platy minerals. - Silicified--replaced by or containing a large amount of quartz or - silica. - Skill facet--a term often used for the pavilion girdle facets of the - standard American brilliant cut (fig. 6). - Specific gravity--the weight in air divided by the loss of weight in - water at a given temperature, or the weight of an object in - air divided by the weight of an equal volume of water; also - called relative density; the most commonly used standard - temperature for this measurement is 4 C. or 39.2 F. - Star facet--one of the eight facets surrounding the table facet of a - standard American brilliant cut (fig. 6). - Step cut--a mode of faceting in which the surface of the gem is - covered by a series of square or rectangular facets; stones - thusly cut are usually square, rectangular, or irregular with - straight sides in outline. - Streak--the color of a mineral when finely powdered; usually - determined by rubbing the mineral against a piece of unglazed - porcelain. - Symmetry--the number, location, and balanced arrangement of crystal - faces in reference to the crystallographic axes or other - crystallographic planes or directions. - Synthetic gem--a gemstone manufactured by man that has approximately - the same chemical composition and properties as a natural - gemstone. - Table facet--the large horizontal facet found on the crown of many - gems, often called simply the table (fig. 6). - Tenacity--the resistance of minerals to breakage, described by such - terms as malleable, ductile, sectile, and brittle (p. 6). - Termination--the end of a crystal that is completely enclosed by - crystal faces, the crystal end that is not attached to the - matrix. - Tertiary--a division of geologic time, estimated to be the time from - 60 to 1 million years ago; the Tertiary includes the - Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs - (from oldest to youngest). - Tetragonal--having four angles; a crystal system in which the crystal - faces are referred to three axes at right angles to each - other, two of which are equal and the third longer or shorter. - Translucent--allowing the passage of light but diffusing it - sufficiently so that objects on the other side cannot be - clearly distinguished. - Transparent--clear, allowing free passage of light so that objects on - the other side can be readily distinguished; opposite of - opaque. - Triclinic--a crystal system in which the crystal faces are referred to - three unequal axes, none of which are at right angles. - Tumbling--a process of polishing irregularly shaped gemstones (p. 17). - Vein--a tabular, irregular, or twisting mineral deposit that is thin - in relation to its length and breadth, usually the result of - solution or hydrothermal activity. - Vitreous--having luster, general appearance, or physical properties - similar to glass. - Vug--an unfilled rock cavity, commonly lined with crystals; may later - become filled by minerals owing to solution or hydrothermal - activity. - - - - - SELECTED REFERENCES - - -Anderson, B. W. (1948) Gem testing: Emerson, New York. - -Baker, C. L. (1935) Metallic and non-metallic minerals and ores -(precious stones), _in_ The geology of Texas, Vol. II, Structural and -economic geology: Univ. Texas Bull. 3401, Jan. 1, 1934, pp. 568-569. - -Barnes, V. E. (1940) North American tektites: Univ. Texas Pub. 3945, -Dec. 1, 1939, pp. 477-582. - -Dake, H. C., Fleener, F. L., and Wilson, B. H. (1938) Quartz family -minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York. - -Ford, W. E. (1932) A textbook of mineralogy (4th ed.): John Wiley and -Sons, Inc., New York. - -Kraus, E. H., and Slawson, C. B. (1947) Gems and gem materials (5th -ed.): McGraw-Hill Book Company, Inc., New York. - -Kunz, G. F. (1892) Gems and precious stones of North America (2d ed.): -Scientific Publishing Company, New York. - -Pough, F. H. (1953) A field guide to rocks and minerals: Houghton -Mifflin Company, Boston. - -Simpson, B. W. (1958) Gem trails of Texas: Granbury, Texas. - -Sinkankas, John (1955) Gem cutting: D. Van Nostrand Company, Inc., -Princeton, New Jersey. - ----- (1959) Gemstones of North America: D. Van Nostrand Company, Inc., -Princeton, New Jersey. - -Smith, G. F. H. (1958) Gemstones (13th ed.), revised by F. C. Phillips: -Methuen and Company, Ltd., London. - -Sperisen, F. J. (1950) The art of the lapidary: The Bruce Publishing -Company, Milwaukee, Wisconsin. - -Sterrett, D. B. (1913) Gems and precious stones, _in_ Mineral resources -of the United States, Calendar Year 1912, Part II, Non-metals: U. S. -Geol. Survey, pp. 1023-1060. - - - - - Plate I - - - [Illustration: A -Gem-quality celestite crystals from Travis County, Texas. Twice natural - size. Lower portion of the crystals is colorless; the tips are dark - blue.] - - [Illustration: B - Opalized wood from the Texas Gulf Coastal Plain. Specimen at left is -rich brown and tan; specimen at right is fossil palm wood and is black, - reddish brown, and white. One-third natural size.] - - - - - Plate II - - - [Illustration: A - Gem-quality garnet crystals and faceted gem from Gillespie County, - Texas. Natural size.] - - [Illustration: B - Labradorite from Brewster County, Texas. Both stones are pale yellow. - One and a half times natural size.] - - - - - Plate III - - - [Illustration: A - Pink microcline crystal from Burnet County, Texas.] - - [Illustration: B -Smoky quartz from Burnet County, Texas. Natural size. Colorless crystal - at center back is included for color comparison.] - - - - - Plate IV - - - [Illustration: Polished agate from gravels of the Rio Grande near -Zapata, Zapata County, Texas. Bands are blue and gray; other inclusions - are brown, yellow, and reddish. One and a half times natural size.] - - - - - Plate V - - - [Illustration: A -Texas tektites (bediasites) showing variety of surface features. Natural - size.] - - [Illustration: B -Topaz crystal from a pegmatite dike near Streeter, Mason County, Texas. - Natural size. Measurements: 1 by 1-5/8 by 3 inches; weight: 194 grams - (970 carats); pale blue; mostly gem quality.] - - - - - Plate VI - - - [Illustration: A - Topaz from stream gravels near Streeter, Mason County, Texas. Natural -size. Left to right: colorless worn pebble; emerald-cut pale-blue topaz, - weight 10 carats; pale-blue worn pebble, weight 205 carats; step out - sky-blue topaz, weight 13 carats; pale-blue worn pebble.] - - [Illustration: B - Tourmaline crystals in schist from Llano County, Texas.] - - - - - Index - - - A - actinolite: 26 - agate: 20, 28, 38 - agatized wood: 27 - allanite: 21 - almandite: 22 - amazonite: 23 - amazon stone: 23 - amber: 18 - amethyst: 25 - Amethyst Hill: 25 - amorphous gemstones: 9 - andalusite: 30 - Arkansas: 19 - Armstrong County: 21 - augite: 18, 28 - - - B - Baringer Hill, Llano County: 21, 26 - baroque pearls and/or stones: 17, 25 - bediasite (tektite): 28-29, 39 - beryl: 18 - Big Bend National Park: 27 - biotite: 23 - Blanco County: 18, 22 - Brazos County: 29 - Brazos River: 25 - Brewster County: 18, 23, 24, 25, 26, 27, 28, 31, 36 - brilliancy: 5 - brilliant cut, standard American: 13, 15, 16 - Brown County: 19 - Burleson County: 29 - Burnet County: 20, 22, 23, 25, 26, 30, 37 - - - C - cabochon gems: 10-12 - Caddo Lake: 25 - carbuncle: 22 - carnelian: 27 - cassiterite: 23, 29 - celestite: 19, 35 - chalcedony: 27 - geodes: 26 - chuck: 15, 17 - citrine: 25-26 - cleavage: 6, 13 - coal: 22 - Coke County: 19 - color: 5 - Colorado River: 25 - Concho River: 25 - crown girdle facets: 16, 17 - crown of gemstone: 15, 16 - crystals: 7-9 - crystal systems: 7 - crytolite: 21 - Culberson County: 22, 23, 25, 26, 31 - culet: 13 - cutting and polishing: 10-17 - cutting lap: 13 - - - D - DeWitt County: 29 - diamond: 19 - saw: 10, 11 - diaphaneity: 5 - dispersion: 6 - dopping: 12, 13 - dop-stick: 12, 15, 17 - dop-wax: 12, 15, 17 - dravite: 30 - durability: 6 - Duval County: 21, 24 - - - E - El Paso County: 22, 30, 31 - emerald cut: 15 - epidote: 19-20 - - - F - facet, kinds of: 13 - main: 16 - skill: 16 - table: 13, 14 - faceted gems and/or stones: 10, 13-17 - Fayette County: 20, 29 - fergusonite: 21 - fire: 5 - Fisher County: 19 - fluorite: 20, 21 - Foard County: 19 - fossil wood: 20-21, 22 - fracture: 6 - Franklin Mountains: 18 - - - G - gadolinite: 21-22 - garnet: 20, 22, 36 - gemstones, by kinds: 18-31 - geodes, celestite: 19 - Georgia: 29 - Gillespie County: 18, 22, 23, 25, 26, 30, 36 - girdle facets: 16 - gneiss: 22 - Gonzales County: 20, 29 - grain: 25 - gram: 7 - graphite: 30 - Grimes County: 29 - grinding: 12 - Guadalupe River: 25 - Gulf Coast: 25 - Gulf Coastal Plain: 18, 20, 22, 35 - - - H - hardness: 6 - Hudspeth County: 18, 20, 22, 24, 25, 26, 27, 28, 31 - - - I - index of refraction: 5 - - - J - jasper: 27-28 - Jeff Davis County: 26, 27, 28, 31 - jet: 22 - - - K - Kinney County: 28 - - - L - labradorite: 23, 36 - Lake Buchanan: 21 - Lampasas County: 19 - lap plate: 13 - Lavaca County: 29 - Lee County: 20, 29 - lignite: 22 - Live Oak County: 21 - Llano County: 18, 19, 20, 21, 22, 23, 25, 26, 30, 31, 40 - Llano River: 25 - luster: 5 - - - M - Madeira topaz: 29 - Mason County: 20, 22, 23, 26, 29, 30, 39, 40 - Massachusetts: 29 - Maverick County: 18 - microcline: 20, 21, 23-24, 29, 37 - Mohs scale of hardness: 6 - moonstone: 28 - Mount Bonnell: 19 - muscovite: 23 - - - N - natural glass: 18, 24, 28 - Needle Peak, Presidio County: 27 - nivenite: 21 - Nolan County: 19 - Nueces River: 25 - - - O - obsidian: 24, 29 - Oklahoma: 29 - opal: 20, 24 - opalized wood: 35 - orbicular jasper: 28 - ounce: 7 - - - P - Packsaddle Mountain: 23 - Packsaddle schist: 30 - palm wood: 21, 35 - Palo Duro Canyon: 21 - pavilion: 13, 16 - facets: 16 - girdle facets: 16, 17 - pearl: 24-25 - pegmatites and/or pegmatite dikes: 18, 20, 21, 22, 23, 26, 29, 39 - petrified wood: 26 - phantom crystals: 26 - pistacite: 20 - pleochroism: 5 - point: 7 - polishing: 17 - lap: 13, 16 - preformed stone: 16 - preforming: 15 - Presidio County: 20, 22, 24, 25, 26, 27, 28 - properties of gemstones: 5-7 - - - Q - quartz: 20, 21, 23, 25-28, 29, 30 - smoky: 38 - - - R - radioactive elements: 22 - radioactivity of gadolinite: 21 - rarity: 6 - Reeves County: 26, 27 - Rio Grande: 25 - gravels of: 21, 27, 38 - Valley: 27 - rock crystal: 26 - rose quartz: 26 - - - S - Sabine River: 25 - sanding: 12 - sanidine: 28 - sawing: 10 - scheelite: 20 - schorl: 30 - size: 7 - "skill" facets: 16 - "slab" of gem materials: 11 - Smithsonian Institution: 21 - smoky quartz: 23, 26, 37 - smoky topaz: 29 - Spanish topaz: 29 - specific gravity: 7 - spinel: 18, 28 - star facets: 17 - step cut: 15 - streak: 6 - synthetic gems: 7 - - - T - table facet: 13, 15 - tektite (bediasite): 28-29, 39 - tenacity: 6 - thorogummite: 21 - topaz: 23, 26, 29-30, 39, 40 - quartz: 29 - tourmaline: 26, 29, 30-31, 40 - Town Mountain, Llano County: 26, 30 - transparency: 6 - Travis County: 19, 35 - Trinity River: 25 - tumbled gems: 17 - turquoise: 31 - - - U - U. S. National Museum: 30 - - - V - value of gemstones: 6, 7 - Val Verde County: 24 - valverdites: 24 - Van Horn, Hudspeth County: 27 - - - W - Walker County: 29 - Washington County: 20 - Webb County: 21 - weight, units of: 7, 25 - - - Z - Zapata County: 38 - - - - - Transcriber's Notes - - ---Silently corrected a few typos. - ---Renumbered figures 6 and 7 (and references to them) to correspond to - their order in the printed book. - ---Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - ---In the text versions only, text in italics is delimited by - _underscores_. - - - - - - - -End of the Project Gutenberg EBook of Texas Gemstones, by Elbert A. 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vertical-align: middle; } -</style> -</head> -<body> - - -<pre> - -The Project Gutenberg EBook of Texas Gemstones, by Elbert A. King, Jr. - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Texas Gemstones - -Author: Elbert A. King, Jr. - -Release Date: August 6, 2019 [EBook #60070] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK TEXAS GEMSTONES *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - -</pre> - -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="Texas Gemstones" width="500" height="738" /> -</div> -<div class="box"> -<p class="center">BUREAU OF ECONOMIC GEOLOGY -<br />The University of Texas at Austin -<br />Austin, Texas 78712</p> -<p class="center">JOHN T. LONSDALE, <i>Director</i></p> -<hr class="dwide" /> -<p class="center"><span class="ss">Report of Investigations—No. 42</span></p> -<hr class="dwide" /> -<h1><span class="larger"><span class="ss">Texas Gemstones</span></span></h1> -<p class="center"><b><span class="small">By</span> -<br /><span class="sc">Elbert A. King, Jr.</span></b></p> -<div class="img"> -<img src="images/i00.jpg" alt="THE UNIVERSITY OF TEXAS AT AUSTIN" width="300" height="303" /> -</div> -<p class="center">February 1961</p> -<p class="center smaller"><i>Second Printing—February 1963 -<br />Third Printing—September 1972 -<br />Fourth Printing—March 1983 -<br />Fifth Printing—August 1991</i></p> -</div> -<div class="pb" id="Page_3">3</div> -<h2>Contents</h2> -<dl class="toc"> -<dt><span class="sc">Page</span></dt> -<dt><a href="#c1">Introduction</a> 5</dt> -<dd><a href="#c2">Properties of gemstones</a> 5</dd> -<dd><a href="#c3">Crystals</a> 7</dd> -<dt><a href="#c4">Cutting and polishing of gemstones</a> 10</dt> -<dd><a href="#c5">Cabochon gems</a> 10</dd> -<dd><a href="#c6">Faceted gems</a> 13</dd> -<dd><a href="#c7">Tumbled gems</a> 17</dd> -<dt><a href="#c8">Texas gemstones</a> 18</dt> -<dd><a href="#c9">Amber</a> 18</dd> -<dd><a href="#c10">Augite</a> 18</dd> -<dd><a href="#c11">Beryl</a> 18</dd> -<dd><a href="#c12">Celestite</a> 19</dd> -<dd><a href="#c13">Diamond</a> 19</dd> -<dd><a href="#c14">Epidote</a> 19</dd> -<dd><a href="#c15">Fluorite</a> 20</dd> -<dd><a href="#c16">Fossil wood</a> 20</dd> -<dd><a href="#c17">Gadolinite</a> 21</dd> -<dd><a href="#c18">Garnet</a> 22</dd> -<dd><a href="#c19">Jet</a> 22</dd> -<dd><a href="#c20">Labradorite</a> 23</dd> -<dd><a href="#c21">Microcline</a> 23</dd> -<dd><a href="#c22">Obsidian</a> 24</dd> -<dd><a href="#c23">Opal</a> 24</dd> -<dd><a href="#c24">Pearl</a> 24</dd> -<dd><a href="#c25">Quartz</a> 25</dd> -<dd class="ddt"><a href="#c26">Crystalline varieties</a> 25</dd> -<dd class="ddt2"><a href="#c27">Amethyst</a> 25</dd> -<dd class="ddt2"><a href="#c28">Citrine</a> 25</dd> -<dd class="ddt2"><a href="#c29">Rock crystal</a> 26</dd> -<dd class="ddt2"><a href="#c30">Rose quartz</a> 26</dd> -<dd class="ddt2"><a href="#c31">Smoky quartz</a> 26</dd> -<dd class="ddt"><a href="#c32">Cryptocrystalline varieties</a> 27</dd> -<dd class="ddt2"><a href="#c33">Chalcedony</a> 27</dd> -<dd class="ddt2"><a href="#c34">Agate</a> 27</dd> -<dd class="ddt2"><a href="#c35">Agatized wood</a> 27</dd> -<dd class="ddt2"><a href="#c36">Carnelian</a> 27</dd> -<dd class="ddt2"><a href="#c37">Jasper</a> 27</dd> -<dd><a href="#c38">Sanidine</a> 28</dd> -<dd><a href="#c39">Spinel</a> 28</dd> -<dd><a href="#c40">Tektite (bediasite)</a> 28</dd> -<dd><a href="#c41">Topaz</a> 29</dd> -<dd><a href="#c42">Tourmaline</a> 30</dd> -<dd><a href="#c43">Turquoise</a> 31</dd> -<dt><a href="#c44">Glossary</a> 32</dt> -<dt><a href="#c45">Selected references</a> 34</dt> -<dt><a href="#c46">Index</a> 41</dt> -</dl> -<h2>Illustrations</h2> -<dl class="toc"> -<dt><span class="jl"><span class="sc">Figures</span>—</span> <span class="sc">Page</span></dt> -<dt><a href="#fig1"><span class="cn">1. </span>Typical crystal form of three common Texas gemstones</a> 9</dt> -<dt><a href="#fig2"><span class="cn">2. </span>Variations of the cabochon cut</a> 10</dt> -<dt><a href="#fig3"><span class="cn">3. </span>Diamond saw</a> 11</dt> -<dt><a href="#fig4"><span class="cn">4. </span>Cabochon properly attached to dop-stick</a> 12</dt> -<dt><a href="#fig5"><span class="cn">5. </span>Cabochons at various stages of cutting and polishing</a> 12</dt> -<dt><a href="#fig6"><span class="cn">6. </span>Nomenclature of the standard American brilliant cut</a> 13</dt> -<dt><a href="#fig7"><span class="cn">7. </span>Facet table</a> 14</dt> -<dt><a href="#fig8"><span class="cn">8. </span>Grinding the table facet on a rough stone</a> 15</dt> -<dt><a href="#fig9"><span class="cn">9. </span>Stone dopped to table facet</a> 15</dt> -<dt><a href="#fig10"><span class="cn">10. </span>Preformed stone dopped to table facet</a> 16</dt> -<dt><a href="#fig11"><span class="cn">11. </span>Proper sequence of cutting of the pavilion facets</a> 16</dt> -<dt><a href="#fig12"><span class="cn">12. </span>Proper placing of pavilion girdle facets</a> 17</dt> -<dt><a href="#fig13"><span class="cn">13. </span>Proper sequence of cutting of crown facets</a> 17</dt> -<dt><a href="#fig14"><span class="cn">14. </span>Common crystal form of Travis County celestite</a> 19</dt> -<dt><a href="#fig15"><span class="cn">15. </span>Common crystal form of fluorite</a> 20</dt> -<dt><a href="#fig16"><span class="cn">16. </span>Crystal faces on microcline specimen shown in Plate III</a> 23</dt> -<dt><a href="#fig17"><span class="cn">17. </span>Common crystal form of spinel</a> 28</dt> -<dt><a href="#fig18"><span class="cn">18. </span>Crystal faces on topaz crystal shown in Plate V</a> 29</dt> -<dt><a href="#fig19"><span class="cn">19. </span>Cross section through irregularly colored stone</a> 30</dt> -<dt><a href="#fig20"><span class="cn">20. </span>Common crystal form of Llano County tourmaline</a> 31</dt> -</dl> -<dl class="toc"> -<dt><span class="jl"><span class="sc">Plates</span>—</span> <span class="sc">Page</span></dt> -<dt><a href="#plateI"><span class="cn">I. </span>A, Gem-quality celestite crystals from Travis County. B, Opalized wood from the Texas Gulf Coastal Plain</a> 35</dt> -<dt><a href="#plateII"><span class="cn">II. </span>A, Gem-quality garnet crystals and faceted gem from Gillespie County. B, Labradorite from Brewster County</a> 36</dt> -<dt><a href="#plateIII"><span class="cn">III. </span>A, Pink microcline crystal. B, Smoky quartz. Both from Burnet County</a> 37</dt> -<dt><a href="#plateIV"><span class="cn">IV. </span>Polished agate from gravels of the Rio Grande near Zapata, Zapata County</a> 38</dt> -<dt><a href="#plateV"><span class="cn">V. </span>A, Texas tektites (bediasites). B, Topaz crystal from a pegmatite dike near Streeter, Mason County</a> 39</dt> -<dt><a href="#plateVI"><span class="cn">VI. </span>A, Topaz from stream gravels near Streeter, Mason County. B, Tourmaline crystals in schist from Llano County</a> 40</dt> -</dl> -<dl class="toc"> -<dt><a href="#table1"><span class="sc">Table 1.</span> Properties of some common Texas gem minerals</a> 8</dt> -</dl> -<div class="pb" id="Page_5">5</div> -<h1 title="">Texas Gemstones</h1> -<p class="center"><b><span class="smaller">ELBERT A. KING, JR.</span></b></p> -<h2 id="c1">INTRODUCTION</h2> -<p>Throughout history man has sought -stones and minerals for personal adornment -and ornamentation. Stones and -minerals that are sufficiently beautiful, -durable, and rare are known as gemstones. -A gemstone with only one of these qualities -is less desirable than one with all three. -For example, a stone with rich color but -not sufficiently durable to withstand daily -wear in rings finds little favor as a gemstone -except in brooches or pins where the -stone is relatively safe from abrasion. Likewise, -a stone that is beautiful and durable -may be of little interest as a gemstone -because it is commonly found in great -quantities. To be valued highly, gemstones -must be beautiful to the eye, durable -enough to withstand wear, and rare enough -so that they are not easily obtained.</p> -<h3 id="c2"><span class="sc">Properties of Gemstones</span></h3> -<p>The beauty of gemstones is mostly dependent -on their color, diaphaneity, brilliancy, -luster, and fire. Any one or a -combination of these properties render -stones desirable as gems.</p> -<p>Color is very important in many gemstones. -The color of transparent varieties -should be distinct enough to be pleasing -to the eye, yet not so dark as to appear -black or opaque. It is generally more desirable -that the gemstone be of even color -and not appear “patchy” or “streaked.” -However, some opaque or translucent -stones such as agate owe their popularity -chiefly to the variety of colors and designs -within a single piece. Some transparent -gemstones exhibit different colors when -viewed in different directions. For example, -some fine blood-red rubies appear -brownish when viewed in a particular -direction. The gemstone should be cut so -that its finest color is most prominently -displayed. This ability of some gemstones -to exhibit different colors when viewed in -different directions is called pleochroism.</p> -<p>Diaphaneity is the relative ability of -stones to transmit light. Diaphaneity is -described by terms such as transparent, -translucent, and opaque. Transparency is -highly desirable in stones such as diamond -that are commonly facet-cut to reflect light. -The gemstone should be water clear and -free from inclusions and cracks so that it -transmits light freely, but there are stones -that do not exhibit this property that are -prized as gemstones. For example, turquoise -may appear to be completely -opaque and not transmit any light, but it -is sought for its fine blue color.</p> -<p>The brilliancy of gemstones is largely -dependent on their index of refraction. -The index of refraction is a measure of -the ability of a cut gemstone to “bend” -light rays and reflect them from the bottom -facets back through the top of the stone. -Of course, brilliancy is not noted in opaque -or faintly translucent stones. The index of -refraction of gemstones is expressed numerically. -Air is the reference medium -and is assigned an index of refraction of -1.00. Other substances are assigned values -relative to that of air, for example, water, -1.33; topaz, 1.62; diamond, 2.42. The -higher the index of refraction, the more -brilliant will be the gemstone if it is -properly cut and polished.</p> -<p>Luster is the appearance of the mineral -on a fresh surface in reflected light; it is -divided into two major categories, metallic -and non-metallic. Most gemstones have -non-metallic luster and are described by -terms such as vitreous or glassy, resinous, -waxy, greasy, and pearly.</p> -<p>The fire, or ability of gemstones to show -flashes of different colors of light, is dependent -<span class="pb" id="Page_6">6</span> -upon a property called dispersion. -The amount of dispersion is the extent to -which the gemstone is able to separate -ordinary white light into its component -colors. The dispersion of gemstones can -also be expressed numerically but for purposes -of this publication will be referred -to as low, moderate, or high. Diamond is -a common gemstone that has high dispersion.</p> -<p>A gemstone’s durability is primarily -dependent upon its hardness. The Mohs -scale of hardness, given below, is most -commonly used for gemstones and other -minerals.</p> -<dl class="undent"><dt class="tu"><b><i>Mohs Scale of Hardness</i></b></dt> -<dt>1. Talc</dt> -<dt>2. Gypsum</dt> -<dt>3. Calcite</dt> -<dt>4. Fluorite</dt> -<dt>5. Apatite</dt> -<dt>6. Orthoclase feldspar</dt> -<dt>7. Quartz</dt> -<dt>8. Topaz</dt> -<dt>9. Corundum</dt> -<dt>10. Diamond</dt></dl> -<p>On this scale, the higher numbers are the -harder minerals. Mohs is a relative, not an -absolute scale. Therefore, it should not be -assumed that diamond is ten times harder -than talc because actually diamond is very -many tens of times harder than talc. However, -a particular mineral is harder than -any other mineral with a lesser number, -and the scale is very convenient to use. -Gemstones mounted in rings should have a -hardness of at least seven on the Mohs -scale, or the stones may become scuffed -and scratched after a relatively short period -of wear. Gemstones mounted in pins -and brooches can be of softer material as -they are not usually subjected to abrasion -and rough treatment.</p> -<p>The tendency of some minerals to split -with relative ease in particular directions -along planes is called cleavage. Cleavage -is also a factor determining the durability -of gemstones. Some gemstones do not exhibit -this tendency at all, whereas others -cleave in several directions. The number -of cleavages is always the same in any one -mineral, and the direction of cleavages is -constant in relation to the crystal structure -of any one mineral or gemstone. It is apparent -that of stones having the same hardness, -the ones lacking cleavage or having -the lesser number of good cleavage directions -are the most durable.</p> -<p>Some stones, such as jade and agate, -owe their durability to their compact -fibrous structure, which makes them very -tough and durable even though they are -not especially hard.</p> -<p>Several other properties of gemstones, -although not always contributing to the -beauty or desirability of gemstones, are -useful in identifying uncut specimens.</p> -<p>Streak is the color of the mineral when -finely powdered or, for softer minerals, -the color obtained by rubbing the mineral -against a piece of unglazed porcelain or -tile. The color of a mineral’s streak is -commonly different from the unpowdered -specimen.</p> -<p>Fracture is the kind of surface obtained -when the mineral is broken in a direction -that is not a cleavage direction. Fracture -surfaces are described by such terms as -conchoidal (like the fracture of glass), -subconchoidal, splintery, even, and uneven.</p> -<p>Tenacity is the resistance of a mineral -to breakage. Brittle minerals break relatively -easily on impact. Malleable minerals, -such as gold, may be flattened under -a hammer into very thin sheets without -breaking. Sectile minerals may be cut with -a knife without powdering. Most gemstones, -even diamond, are brittle.</p> -<p>It is only natural to value most those -gemstones that are not common or easy to -obtain. Emerald owes its longstanding -popularity to its fine green color, but tourmaline -is sometimes found in colors that -very closely approach that of emerald and -yet sells for considerably less because it is -so much more common.</p> -<p>Rarity is not the only factor affecting -the value of gemstones. Freedom from internal -imperfections, quality of cutting, -color, and size must also be considered in -cut and polished gemstones. Internal imperfections, -such as inclusions and cracks, -<span class="pb" id="Page_7">7</span> -detract from the appearance of gemstones -and interfere with the passage of light between -the facets; consequently, gemstones -containing these imperfections are not -valued as highly as those without them. -Poor cutting or polishing detract from the -beauty and thus from the value of gemstones. -Unpopular or poor color commonly -causes gemstones to be less valuable. Rich -green emeralds are exceedingly prized, -whereas very pale green emeralds are -relatively inexpensive. Diamonds that have -the least hint of yellow are never valued -as highly as pure colorless, pink, or blue -stones. Few persons find the yellowish color -attractive, unless it is a vivid canary -yellow.</p> -<p>Size is important in determining the -value of gemstones but not as important -as perfection. A badly flawed gemstone of -large size may be worth only a slight fraction -of the value of a smaller perfect one. -Gemstone size is usually measured in -carats, a unit of weight, although millimeter -size is sometimes used. Five carats -is equal to 1 gram and approximately -28⅓ grams is equal to 1 ounce avoirdupois. -One one-hundredth (0.01) of a carat -is called a point, and this term is often -used, especially pertaining to very small -gemstones.</p> -<p>The term used to compare the relative -weights of minerals and gemstones is -specific gravity, which is expressed numerically -in relation to water. Water is -assigned the value of 1.00. Therefore, at -a given temperature a gemstone having a -specific gravity of 2.00 is twice as heavy -as an equal volume of water. A 1-carat -sapphire (specific gravity about 4.00) -will be smaller than a 1-carat amethyst -(specific gravity about 2.65) because the -heavier material will occupy less volume -to have the same weight.</p> -<p>A summary of properties helpful in -identification of common Texas gem -minerals is given in <a href="#table1">Table 1</a>.</p> -<p>Comparatively recently in the history of -gemstones, man has succeeded in the -production of synthetic gems that have -properties closely approaching those of -many natural gemstones. To the untrained -eye some synthetic gems may appear -identical to natural stones, but synthetic -gems can be detected with little difficulty -by a properly equipped expert. Although -most synthetic gems are inexpensive, their -manufacture has not adversely affected the -value of natural gemstones but instead -has increased the demand for fine natural -gems.</p> -<h3 id="c3"><span class="sc">Crystals</span></h3> -<p>Gemstones that have an orderly internal -molecular arrangement are referred to as -crystalline. This internal order is commonly -reflected in the external shape of -“rough” or uncut gemstones. The resultant -shape is a polyhedral solid bounded by -planes and called a crystal. Well-formed -crystals are formed in nature only under -relatively ideal conditions of temperature, -pressure, and space. The specific temperatures -and pressures involved vary with -different minerals, but most crystals need -space in which to form so that their -“growth” is not impaired by surrounding -rocks and minerals. However, some minerals, -such as garnet and tourmaline, can -grow in metamorphic rocks by recrystallization -of minerals in the metamorphic -rocks. The size of crystals varies from -microscopic to tens of feet. Any one -mineral usually has one or two typical -crystal forms or arrangements of plane -surfaces that aid greatly in the identification -of the mineral when it occurs in good -crystals (<a href="#fig1">fig. 1</a>). Frequently gemstones -are found as abraded stream-rolled pebbles, -fragments, or masses that do not -show crystal form. Crystals of the same -mineral from different locations commonly -show somewhat different crystal -forms owing to slight differences in composition -or conditions of formation. -Mineralogists and crystallographers classify -crystals by the symmetry that they -exhibit. The crystal systems are (1) isometric -or cubic, (2) tetragonal, (3) hexagonal, -(4) orthorhombic, (5) monoclinic, -and (6) triclinic. A complete description -of the classification of crystals can be -found in almost any mineralogy text (see -Selected References, p. 34).</p> -<div class="pb" id="Page_8">8</div> -<table class="center"> -<tr class="th"><th id="table1" colspan="6"><span class="sc">Table 1.</span> Properties of some common Texas gem minerals.</th></tr> -<tr class="th"><th>MINERAL </th><th>COMPOSITION </th><th>HARDNESS </th><th>SPECIFIC GRAVITY </th><th>INDEX OF REFRACTION </th><th>COMMON COLORS IN TEXAS</th></tr> -<tr><td class="l">Amber </td><td class="c">fossil resin </td><td class="c">2.0-2.5 </td><td class="c">1.05-1.10 </td><td class="c">about 1.54 </td><td class="c">brown, yellow</td></tr> -<tr><td class="l">Augite </td><td class="c">CaMgSi₂O₆ </td><td class="c">5.0-6.0 </td><td class="c">3.2-3.6 </td><td class="c">1.60-1.71 </td><td class="c">greenish brown, black</td></tr> -<tr><td class="l">Beryl </td><td class="c">Be₃Al₂(SiO)₆ </td><td class="c">7.5-8.0 </td><td class="c">2.63-2.80 </td><td class="c">1.56-1.60 </td><td class="c">pale blue, colorless, greenish</td></tr> -<tr><td class="l">Celestite </td><td class="c">SrSO₄ </td><td class="c">3.0-3.5 </td><td class="c">3.95-3.98 </td><td class="c">1.62-1.63 </td><td class="c">colorless, blue</td></tr> -<tr><td class="l">Epidote </td><td class="c">HCa₂(Al, Fe)₃Si₃O₁₃ </td><td class="c">6.0-7.0 </td><td class="c">3.25-3.50 </td><td class="c">1.72-1.77 </td><td class="c">yellowish green, brownish green</td></tr> -<tr><td class="l">Fluorite </td><td class="c">CaF₂ </td><td class="c">4.0 </td><td class="c">3.0-3.25 </td><td class="c">1.434 </td><td class="c">colorless, violet, yellow, green</td></tr> -<tr><td class="l">Garnet (Almandite) </td><td class="c">Fe₃Al₂(SiO₄)₃ </td><td class="c">about 7.5 </td><td class="c">4.25 </td><td class="c">about 1.83 </td><td class="c">red, deep red, brownish red</td></tr> -<tr><td class="l">Labradorite </td><td class="c">NaAlSi₃O₈ 50% to 30% CaAlSi₃O₈ 50% to 70% </td><td class="c">6.0-6.5 </td><td class="c">about 2.6 </td><td class="c">about 1.56 </td><td class="c">yellowish, grayish</td></tr> -<tr><td class="l">Microcline </td><td class="c">KAlSi₃O₈ </td><td class="c">6.0-6.5 </td><td class="c">2.54-2.57 </td><td class="c">1.52-1.53 </td><td class="c">pink, red, bluish, greenish, white</td></tr> -<tr><td class="l">Obsidian </td><td class="c">volcanic glass </td><td class="c">5.0-5.5 </td><td class="c">2.3-2.5 </td><td class="c">1.45-1.53 </td><td class="c">dark gray, black, brownish</td></tr> -<tr><td class="l">Opal </td><td class="c">SiO₂·nH₂O </td><td class="c">5.5-6.5 </td><td class="c">1.9-2.3 </td><td class="c">1.43 </td><td class="c">white, pink, bluish, brown, gray</td></tr> -<tr><td class="l">Quartz (Crystalline) </td><td class="c">SiO₂ </td><td class="c">7.0 </td><td class="c">2.65-2.66 </td><td class="c">1.544-1.553 </td><td class="c">colorless, violet, yellow, brown</td></tr> -<tr><td class="l">Tektite (Bediasite) </td><td class="c">natural glass </td><td class="c">5-6 </td><td class="c">2.33-2.44 </td><td class="c">1.48-1.52 </td><td class="c">dark brown, greenish brown</td></tr> -<tr><td class="l">Topaz </td><td class="c">Al₂(F·OH)₂SiO₄ </td><td class="c">8.0 </td><td class="c">3.4-3.6 </td><td class="c">1.60-1.63 </td><td class="c">colorless, bluish, sky blue</td></tr> -<tr><td class="l">Tourmaline </td><td class="c">H₉Al₃(B·OH)₂Si₄O₁₉ </td><td class="c">7.0-7.5 </td><td class="c">2.98-3.20 </td><td class="c">1.62-1.64 </td><td class="c">black, dark brown</td></tr> -</table> -<div class="pb" id="Page_9">9</div> -<p>Some gemstones, such as opal and -obsidian, never occur as crystals owing to -a lack of internal structural order. Such -gemstones are termed amorphous, or without -form. Amorphous gemstones mostly -occur in nature as irregular lumps or -masses, cavity fillings, or veins.</p> -<div class="img" id="fig1"> -<img src="images/i01.jpg" alt="" width="800" height="352" /> -<p class="caption"><span class="sc">Fig. 1.</span> Typical crystal form of three common Texas gemstones.</p> -</div> -<dl class="undent pcap"><dt>GARNET</dt> -<dt>TOURMALINE</dt> -<dt>QUARTZ</dt></dl> -<div class="pb" id="Page_10">10</div> -<h2 id="c4">CUTTING AND POLISHING OF GEMSTONES</h2> -<p>There are two types of widely used gemstone -cuts. Opaque or figured gemstones -are usually cut with a rounded upper surface -and a flat or rounded back. A stone -cut in this fashion is termed a cabochon -or is said to be cabochon cut. There are -several variations of this mode of cutting -(<a href="#fig2">fig. 2</a>). Precious opal, agate, jade, star -sapphire, and fossil wood are some of the -stones that are cut mostly as cabochons. -Transparent gemstones are usually cut -with many plane polished surfaces. Such -stones are called faceted, and the process -of cutting and polishing these stones is -called faceting. Emerald, diamond, topaz, -and garnet are examples of gemstones that -are commonly seen as faceted stones.</p> -<div class="img" id="fig2"> -<img src="images/i02.jpg" alt="" width="700" height="60" /> -<p class="caption"><span class="sc">Fig. 2.</span> Variations of the cabochon cut. Left to right: double cabochon; flat cabochon; simple cabochon; hollow cabochon.</p> -</div> -<p>The cutting of gemstones, although -sometimes tedious and time consuming, is -not especially difficult or complex. However, -like most arts and crafts, technique -and ability should improve with practice -and experience. There are currently many -amateur gem cutters in Texas. A complete -set of equipment necessary to cut cabochon -stones may be purchased for as little -as $50.00 or $60.00. Most amateur cabochon -cutters have equipment that cost less -than $100.00 which enables them to do -very fine work on many gem materials. -Facet cutting requires more precise equipment, -and a complete array of such usually -costs more than $100.00, although less -expensive equipment can be obtained. The -beginning gem cutter or lapidary who is -willing to assemble and make some of his -own equipment can reduce his initial expenses -considerably.</p> -<h3 id="c5"><span class="sc">Cabochon Gems</span></h3> -<p>The procedures listed herein for gem -cutting do not apply to all gemstones. -Stones that are especially brittle, soft, or -difficult to polish require additional procedures -or special techniques. Many lapidaries -may deviate from these procedures. -Some of the steps of cutting and polishing -are merely matters of personal opinion -and vary somewhat from cutter to cutter. -There are several detailed texts on the art -of gem cutting; the descriptions herein -are designed to give the reader only a -general idea of the procedures and techniques -involved.</p> -<p>The cutting and polishing of cabochons -require several steps. The initial step is -sawing. Assuming that the rough gem -material is large enough to be sawed -(larger than about half an inch in -diameter), it is clamped into the carriage -of a diamond saw (<a href="#fig3">fig. 3</a>) and cut into -slices about ⅜-inch thick. The blade of -the saw is mild steel that has been impregnated -with diamond dust around the -edge, hence the name diamond saw. The -blade is rotated rapidly, and the material -to be cut is “fed” to the blade by a sliding -carriage on which the gem material is -clamped. The extreme hardness of the -diamond dust in the edge of the blade -enables the saw to cut through several -inches of gem material in a few minutes. -The lower portion of the saw blade is -immersed in a mixture of kerosene and -oil, and the rotating saw blade carries with -it some of the kerosene-oil mixture; this -acts as a coolant and lubricant for both -the saw blade and the material being cut. -Without this lubricant, the heat generated -by sawing would shatter most gem materials -and also damage the saw blade. As -this “slicing” or sawing of the material -usually takes several minutes, a weight -<span class="pb" id="Page_11">11</span> -and pulley are generally used to give the -gem material the necessary pressure -against the saw blade. When cut through, -the “slab” of gem material falls into the -kerosene-oil mixture at the bottom of the -saw or onto a special platform that cushions -its fall.</p> -<div class="img" id="fig3"> -<img src="images/i02a.jpg" alt="" width="700" height="573" /> -<p class="caption"><span class="sc">Fig. 3.</span> Diamond saw.</p> -</div> -<dl class="undent pcap"><dt>Motor</dt> -<dt>Clamp</dt> -<dt>Diamond-charged blade</dt> -<dt>Carriage</dt> -<dt>Stone</dt> -<dt>Weight</dt></dl> -<p>After being sawed, the slab of gem material -is examined, and the location and -size of the stones to be cut from the slab -are determined. The desired outline of the -shape of the gem to be cut is marked on -the slab with a pointed piece of aluminum -rod; ordinary pencil marks are not used -because they wear away too quickly in the -cutting process. Once the area from which -the gem is to be cut has been selected and -the outline of the gemstone has been -marked on the slab, the excess material is -trimmed away by a smaller diamond saw -known as a trim-saw. In some slabs the -excess material can be broken and -“nibbled” away with a strong pair of -pliers.</p> -<p>The remaining portion of the stone is -usually held by hand and ground to the -desired shape using the previously scribed -mark as a guide. This is done using a -relatively coarse-grained (about 150 grit) -specially made carborundum grinding -wheel.</p> -<p>Now that the desired outline has been -obtained, the stone is firmly affixed to a -slender wooden or hollow aluminum dop-stick -<span class="pb" id="Page_12">12</span> -(<a href="#fig4">fig. 4</a>). The process whereby the -stone is attached to the dop-stick with a -specially compounded jeweler’s wax is -called dopping. The dop-wax is heated -over an alcohol lamp or candle flame until -it is soft and pliable and is then spread -around on the end of the dop-stick and -formed into a mass about the right size -and shape to fit the back of the gemstone. -The stone is likewise heated, and the wax -is applied to the back of the stone while -both wax and stone are hot. Upon cooling, -the wax firmly fixes the stone to the dop-stick. -The dop-stick allows the lapidary to -have firm control of the stone during all -later stages of cutting and polishing.</p> -<div class="img" id="fig4"> -<img src="images/i03.jpg" alt="" width="500" height="317" /> -<p class="caption"><span class="sc">Fig. 4.</span> Cabochon properly attached to dop-stick.</p> -</div> -<dl class="undent pcap"><dt>CABOCHON</dt> -<dt>DOP-WAX</dt> -<dt>DOP-STICK</dt></dl> -<p>The top of the dopped gemstone is -worked against the coarse carborundum -grinding wheel until it is a rough approximation -of the desired shape. The stone is -then worked against a much finer-grained -(about 220 grit) grinding wheel to remove -the irregularities left by the coarse grinding -and to further smooth and shape the -surface of the gemstone. At all times while -grinding, a small flow of water should be -directed on the grinding wheel to keep the -stone cool. Grinding on the stone for even -a few minutes without cooling may result -in the shattering of the gemstone because -of heat created by friction of the stone -against the grinding wheel. If the lapidary -keeps the surface of the grinding wheel -wet, there is little chance of damaging -most gem materials.</p> -<p>The next phase of cabochon cutting and -polishing is sanding. The gemstone is -worked against two sanding drums of -different grit size. This sanding can be -done with the sandpaper surface either -wet or dry, as needed or as preferred by -the lapidary. However, great care should -be exercised during sanding so that the -stone is not overheated. Overheating can -easily occur whether the sandpaper is -used wet or dry. As in grinding, sanding -is first done on coarser grit paper (about -300 grit) and last on finer paper (about -600 grit). It is in the sanding process that -the first hint of polish is noted on the surface -of the stone. After sanding, the gemstone -should have perfect form with no -surface irregularities, a very finely textured -surface, and only very minor -scratches left from sanding. The gemstone -is now ready to be polished.</p> -<div class="img" id="fig5"> -<img src="images/i03a.jpg" alt="" width="700" height="155" /> -<p class="caption"><span class="sc">Fig. 5.</span> Cabochons at various stages of cutting and polishing. Left to right: trimmed from slab: ground to outline; after rough grinding; after sanding; polished.</p> -</div> -<p>At this point the procedure depends on -the nature of the gemstone being polished. -Most gem materials are worked against a -buffing wheel that is impregnated or saturated -with a mixture of some polishing -compound and water. A soft felt buffing -wheel with cerium oxide as the polishing -agent is used for many materials. The -mixture of cerium oxide and water is -usually applied to the buffing wheel with -a small brush. The lapidary should once -more be careful not to overheat the stone. -If the stone becomes too hot to hold to the -<span class="pb" id="Page_13">13</span> -underside of the cutter’s wrist, it should -be permitted to cool for a few seconds -before continuing. After polishing on the -buffing wheel, the gemstone should have -a fine, high polish and be free of any -scratches or surface irregularities. The -finished gemstone is removed from the -dop-stick by heating the dop-wax and pulling -the stone loose. Any excess wax that -hardens again before it can be removed -from the stone by hand can be dissolved -away by rubbing with an acetone-soaked -cloth. <a href="#fig5">Figure 5</a> illustrates the desired appearance -of the gemstone at the end of -each of the steps of cutting and polishing.</p> -<h3 id="c6"><span class="sc">Faceted Gems</span></h3> -<p>The principles involved in faceting are -about the same as those in the cutting of -cabochons, but the equipment and technique -are considerably different. The -equipment required for the facet cutting -of gemstones is built into or attached to -a small specially constructed table (<a href="#fig7">fig. 7</a>), -and the unit is commonly called a facet -table. Most faceted gemstones are cut to -obtain the largest flawless stone possible -from the rough material. Therefore, one -of the first and most important steps for -the lapidary is to decide how the stone is -to be cut from the rough crystal or pebble. -The colors that can be obtained from the -gemstone must also be considered, and the -cutting of the stone oriented so that its best -color is displayed. The lapidary also -selects the orientation of the stone in relation -to the cleavage or cleavages. It is -difficult or impossible to polish facets of -gemstones that are cut parallel to a good -cleavage direction.</p> -<div class="img" id="fig6"> -<img src="images/i03c.jpg" alt="" width="458" height="800" /> -<p class="caption"><span class="sc">Fig. 6.</span> Nomenclature of the standard American brilliant cut.</p> -</div> -<dl class="undent pcap"><dt>TOP VIEW</dt> -<dt>SIDE VIEW</dt> -<dd>Star facet</dd> -<dd>Crown main facet</dd> -<dd>Crown girdle facet</dd> -<dd>Pavilion girdle facet</dd> -<dd>Pavilion main facet</dd> -<dd>TABLE</dd> -<dd>CROWN GIRDLE</dd> -<dd>PAVILION</dd> -<dd>CULET</dd> -<dt>BOTTOM VIEW</dt></dl> -<p>Once the orientation of the gemstone to -be cut from the rough material has been -determined, the stone is dopped onto a -special metal dop-stick that fits into the -chuck of the facet head. The chuck is -tightened so that the position of the stone -on the end of the arm of the facet head is -firmly fixed, and the facet head is adjusted -so that the first facet that is cut is -the horizontal, top facet of the stone or -table facet (<a href="#fig6">fig. 6</a>). The table facet is cut -by grinding the gemstone on a flat cutting -lap that is diamond impregnated (<a href="#fig8">fig. 8</a>). -By minor adjustments of the facet head, -the lapidary can precisely control the location -of the table facet. As soon as the -table facet has been ground to the proper -size, the cutting lap is removed from the -lap plate, and the polishing lap is secured -in place. Many different kinds of polishing -laps and polishing compounds may be -used depending on the properties of the -material being polished. However, one lap -and one polishing compound are usually -sufficient for each gem variety. After the -polishing lap is secured to the lap plate, -the lapidary adjusts the facet head so that -the stone is in exactly the same position -relative to the lap that it was during the -cutting of the table facet. The polishing -lap is run wet or damp with water, as is -the cutting lap, and small amounts of the -polishing compound are applied to the -surface of the lap while the facet is being -polished. The minor scratches left by the -cutting process are gradually removed, -and a fine lustrous polish develops on the -facet. It is especially important to take -care in achieving a perfect polish on the -table facet, as this facet occupies a large -area of the crown of the gemstone. When -the cutting and polishing of the table facet -are completed, the gemstone is still rough -or uncut in all portions except for this -single, large, polished surface.</p> -<div class="pb" id="Page_14">14</div> -<div class="img" id="fig7"> -<img src="images/i04.jpg" alt="" width="800" height="616" /> -<p class="caption"><span class="sc">Fig. 7.</span> Facet table.</p> -</div> -<dl class="undent pcap"><dt>Water</dt> -<dt>Light</dt> -<dt>Adjusting ring</dt> -<dt>Post</dt> -<dt>Arm</dt> -<dt>Chuck</dt> -<dt>Stone</dt> -<dt>Abrasives</dt> -<dd>DIAMOND DUST</dd> -<dd>CALCIUM OXIDE</dd> -<dd>LANDE-A</dd></dl> -<div class="pb" id="Page_15">15</div> -<div class="img" id="fig8"> -<img src="images/i04a.jpg" alt="" width="700" height="552" /> -<p class="caption"><span class="sc">Fig. 8.</span> Grinding the table facet on a rough stone.</p> -</div> -<dl class="undent pcap"><dt>CHUCK</dt> -<dt>DOP-STICK</dt> -<dt>DOP-WAX</dt> -<dt>STONE</dt> -<dt>LAP</dt></dl> -<p>The gemstone is then removed from the -dop-stick by melting the dop-wax and is -dopped once more so that the plane of the -polished table facet is perpendicular to -the axis of the chuck and arm of the facet -head (<a href="#fig9">fig. 9</a>). Great care should be taken -by the lapidary to insure that the table of -the stone is exactly perpendicular to this -axis, or the proper placing of the later -facets on the stone may become very difficult.</p> -<div class="img" id="fig9"> -<img src="images/i04c.jpg" alt="" width="400" height="261" /> -<p class="caption"><span class="sc">Fig. 9.</span> Stone dopped to table facet.</p> -</div> -<dl class="undent pcap"><dt>TABLE FACET</dt> -<dt>DOP-WAX</dt> -<dt>STONE</dt> -<dt>DOP-STICK</dt></dl> -<p>Once the stone has been properly dopped -to the table facet, the lapidary is ready to -proceed with the cutting of the outline of -the stone. If it is to be a brilliant cut, the -stone is ground perfectly round in outline; -if it is to be an emerald or step cut, it is -shaped so that it is square or rectangular -in outline. This process is called preforming. -The arm of the facet head is lowered -on the post until it is horizontal, and the -stone is worked against the cutting lap -until the desired shape is obtained. When -<span class="pb" id="Page_16">16</span> -the preforming process is completed, the -stone should have the desired outline of -the finished gem (<a href="#fig10">fig. 10</a>).</p> -<div class="img" id="fig10"> -<img src="images/i05.jpg" alt="" width="400" height="221" /> -<p class="caption"><span class="sc">Fig. 10.</span> Preformed stone dopped to table facet.</p> -</div> -<dl class="undent pcap"><dt>DOP-WAX</dt> -<dt>STONE</dt> -<dt>DOP-STICK</dt></dl> -<p>The lapidary is now ready to proceed -with the cutting of the pavilion of the stone. -The arm of the facet head is raised to the -proper angle for cutting the main pavilion -facets. The angle at which the main facets -are cut is very critical in determining the -beauty of the finished stone. The required -angle at which these facets must be cut -varies with the refractive indices of the -different varieties of gem minerals. If the -facets are not cut at exactly the proper -angle, light entering the top or crown of -the gemstone can pass completely through -the stone, instead of being reflected back -out of the crown facets. The result is a -dull, lifeless stone that appears to have a -“hole” or “fish-eye” in the center. Stones -that are cut in this manner are greatly -reduced in value. The angle at which the -facets are cut is controlled by the adjustment -of the height of the arm of the facet -head on the post. The lapidary will continually -adjust this height, because the -angle between the arm and the surface of -the lap changes slightly as the facet is -ground down to its proper place and size.</p> -<div class="img" id="fig11"> -<img src="images/i05a.jpg" alt="" width="800" height="193" /> -<p class="caption"><span class="sc">Fig. 11.</span> Proper sequence of cutting of the pavilion facets. Left to right: four main facets; all eight main facets; half of the pavilion girdle facets; completed pavilion.</p> -</div> -<p>The standard American brilliant cut -will be used as an example of facet cutting. -Procedure for all other cuts is essentially -the same to this point. After the eight main -pavilion facets have been cut, the cutting -angle is changed a few degrees, the arm -of the facet head rotated slightly, and the -sixteen pavilion girdle facets or “skill” -facets, as they are often called, are cut -(<a href="#fig11">fig. 11</a>). The pavilion girdle facets should -meet exactly in the center of the main -facets at the girdle of the stone. The pavilion -girdle facets should neither overlap, -nor should there be any space between -them (<a href="#fig12">fig. 12</a>). After the pavilion girdle -facets are cut, the cutting of the pavilion -of the gemstone is completed. The facets -are then polished on the polishing lap at -the same angles and in the same order as -they were cut, and the pavilion of the gem -is completely finished.</p> -<p>The stone is then removed from the dop-stick -by melting the dop-wax and is re-dopped -to the pavilion facets so that the -crown of the stone is now exposed for cutting. -Before the lapidary proceeds with -the cutting of the crown, it is necessary -that the stone be perfectly centered on the -dop-stick and that the plane of the table -facet be perpendicular to the dop-stick and -to the axis of the arm of the facet head. -The eight main facets are cut first, with -numerous adjustments being made by the -lapidary to insure that the proper angle is -maintained (<a href="#fig13">fig. 13</a>). Then the cutting -angle is changed a few degrees, the arm of -the facet head rotated slightly, and the -crown girdle facets are cut. The crown -girdle facets are placed very similarly to -the pavilion girdle facets except that they -<span class="pb" id="Page_17">17</span> -are shorter. The crown girdle facets should -be joined in exactly the same way as the -pavilion girdle facets. When these facets -are properly cut, the cutting angle is again -changed, the arm rotated, and the eight -star facets are cut. This completes the cutting -of the crown of the stone. The cutting -lap is removed from the lap plate, and the -polishing lap is secured into place. The -facets are carefully polished in the same -order that they were cut. After the last star -facet has been polished, the stone is removed -from the dop-stick. Any excess dop-wax -is removed from the stone by means -of a solvent, and the full beauty of the -finished gem is revealed.</p> -<h3 id="c7"><span class="sc">Tumbled Gems</span></h3> -<p>One other method of finishing gemstones -that deserves mention is tumbling. “Baroque” -or “free-form” stones are produced -in this manner. Loose pebbles or pieces of -gem materials left over from other cutting -processes are placed in a small barrel or -specially constructed box with loose carborundum -grit. The barrel is turned by -means of a small motor, and the abrasion -of the pebbles and grit against each other -tends to round the pebbles and give them -a finely pitted surface. Progressively finer -and finer carborundum grit is used, and -eventually a polishing compound. The -result is several pounds of well-polished -gem pebbles of various shapes and sizes. -These baroque stones have found recent -favor in costume jewelry of modern design. -The tumbling process is rather slow, commonly -requiring several days or weeks. -However, little effort is involved on the -part of the lapidary, and, consequently, -the cost of most tumbled or baroque stones -is quite modest. Only gem material that is -unsuitable for cutting in other manners -should be finished in this way.</p> -<div class="img" id="fig12"> -<img src="images/i05c.jpg" alt="" width="800" height="323" /> -<p class="caption"><span class="sc">Fig. 12.</span> Proper placing of the pavilion girdle facets. Left: facets not joined. Center: facets overlapped, joined too high. Right: correct placing.</p> -</div> -<dl class="undent pcap"><dt>Stone</dt> -<dt>Dop-wax</dt> -<dt>Dop-stick</dt> -<dt>Chuck</dt></dl> -<div class="img" id="fig13"> -<img src="images/i05d.jpg" alt="" width="738" height="175" /> -<p class="caption"><span class="sc">Fig. 13.</span> Proper sequence of cutting of the crown facets. Left to right: four main facets; all eight main facets; half of the crown girdle facets; completed crown.</p> -</div> -<div class="pb" id="Page_18">18</div> -<h2 id="c8">TEXAS GEMSTONES</h2> -<h3 id="c9"><span class="sc">Amber</span></h3> -<blockquote> -<p><i>Composition</i>: fossil resin. <i>Crystal system</i>: amorphous. -<i>Hardness</i>: about 2.0 to 2.5. <i>Specific gravity</i>: -variable, from 1.05 to 1.10. <i>Luster</i>: resinous. -<i>Color</i>: brown, yellow, red, orange, and white. -<i>Streak</i>: white to yellowish to gray. <i>Cleavage</i>: -none. <i>Fracture</i>: conchoidal. <i>Tenacity</i>: brittle. -<i>Diaphaneity</i>: transparent to translucent. <i>Refractive -index</i>: variable, about 1.54. Burns with a -sweet, piney odor.</p> -</blockquote> -<p>Rich brown to yellowish amber has been -found near Eagle Pass, Maverick County, -in Cretaceous coal and on Terlingua Creek, -Brewster County. Although much of this -material is translucent and the quality -suitable for lapidary purposes, the pieces -are seldom more than a fraction of an inch -in diameter.</p> -<p>Occasional finds of poor quality brownish -amber have been reported from the -Tertiary formations of the Gulf Coastal -Plain, but thus far no gem quality material -has been found.</p> -<p>The softness of amber limits its use to -brooches, necklaces, and other jewelry that -is relatively safe from abrasion.</p> -<h3 id="c10"><span class="sc">Augite</span></h3> -<blockquote> -<p><i>Composition</i>: CaMgSi₂O₄; may also contain -iron, aluminum, and sometimes titanium. <i>Crystal -system</i>: monoclinic. <i>Hardness</i>: 5 to 6. <i>Specific -gravity</i>: 3.2 to 3.6. <i>Luster</i>: vitreous to dull. <i>Color</i>: -dark greenish brown and greenish black. <i>Streak</i>: -light grayish green. <i>Cleavage</i>: two directions, -poor. <i>Fracture</i>: conchoidal to uneven. <i>Tenacity</i>: -brittle. <i>Diaphaneity</i>: opaque to translucent. <i>Refractive -index</i>: variable, about 1.60 to 1.71.</p> -</blockquote> -<p>Augite of gem quality occurs near Eagle -Flat, Hudspeth County, Texas. Although -this material is very dark greenish brown -and not commonly thought of as a gemstone, -lapidaries have used it to fashion -black faceted stones and cabochons that -resemble obsidian. Most of the augite -occurs as loose pieces and crystal fragments -that have weathered out of nearby -igneous rocks; the augite can also be found -in situ in the igneous rocks.</p> -<p>Specimens and pieces of cutting quality -1 inch in diameter are common, and fragments -over 2 inches in diameter have been -found. The augite is associated with black -spinel and some dark gray to black pieces -of natural glass. Although the faceted and -cabochon-cut stones are not particularly -attractive, some of the larger pieces of -augite might be utilized for carving.</p> -<h3 id="c11"><span class="sc">Beryl</span></h3> -<blockquote> -<p><i>Composition</i>: Be₃Al₂(SiO)₆. <i>Crystal system</i>: -hexagonal. <i>Hardness</i>: 7.5 to 8.0. <i>Specific gravity</i>: -2.63 to 2.80. <i>Luster</i>: vitreous. <i>Color</i>: pale blue, -blue, green, yellow, brownish, pink, and colorless. -<i>Streak</i>: white. <i>Cleavage</i>: one direction, very imperfect. -<i>Fracture</i>: conchoidal to uneven. <i>Tenacity</i>: -brittle. <i>Diaphaneity</i>: transparent to subtranslucent. -<i>Refractive index</i>: 1.56 to 1.60. <i>Dispersion</i>: -low.</p> -</blockquote> -<p>Gem-quality beryl has not been reported -in Texas. A discussion of beryl is included -herein because the writer believes it likely -that beryl of gem quality will be found in -Texas as a result of future investigations -and exploration.</p> -<p>Beryl crystals have been found in pegmatite -dikes in Llano, Blanco, and Gillespie -counties. These crystals are commonly -several inches long and exceed 1 inch in -diameter but are very badly fractured. -Most of the beryl crystals do not approach -gem quality and are entirely unsuitable -for any lapidary use. The color of the -crystals found thus far is bluish, greenish, -pinkish brown, yellowish, and colorless. -Some very tiny colorless beryl crystals -have been found that are transparent, but -thus far such crystals have been too small -to be cut into gems.</p> -<p>Fine blue beryl crystals have been found -in the Franklin Mountains near El Paso, -Texas. Unfortunately, these crystals are so -badly flawed and fractured that they are -not suitable for lapidary use.</p> -<p>It seems likely that careful prospecting -of Texas pegmatites will reveal at least -some gem-quality beryl.</p> -<div class="pb" id="Page_19">19</div> -<h3 id="c12"><span class="sc">Celestite</span></h3> -<blockquote> -<p><i>Composition</i>: SrSO₄. <i>Crystal system</i>: orthorhombic. -<i>Hardness</i>: 3.0 to 3.5. <i>Specific gravity</i>: -3.95 to 3.98. <i>Luster</i>: vitreous. <i>Color</i>: white, blue, -greenish, reddish, and brownish. <i>Streak</i>: white. -<i>Cleavage</i>: three directions, although one of these -directions is not easily developed. <i>Fracture</i>: uneven. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: transparent -to subtranslucent. <i>Refractive index</i>: 1.62 to 1.63. -<i>Dispersion</i>: moderate.</p> -</blockquote> -<p>Celestite is very seldom cut into gems. -Being very soft, brittle, and having three -cleavages, celestite is completely unsuitable -for jewelry. These same properties -make this mineral exceedingly difficult to -facet; however, faceted stones are seen in -large collections.</p> -<div class="img" id="fig14"> -<img src="images/i06.jpg" alt="" width="167" height="300" /> -<p class="caption"><span class="sc">Fig. 14.</span> Common crystal form of Travis County celestite. Same crystal form as shown in <a href="#plateI">Plate I</a>, A.</p> -</div> -<p>Fine crystals of colorless and blue gem-quality -celestite (<a href="#plateI">Pl. I</a>, A, and <a href="#fig14">fig. 14</a>) have -been found at Mount Bonnell and other -localities west of Austin, Travis County. -The celestite crystals occur in vugs or -geodes in limestone. The crystals are -mostly white or colorless and fractured -near the base or where attached, but the -tips of the crystals are commonly clear -celestine blue and completely free of -flaws.</p> -<p>Crystals several inches in length have -been found, but the average size is about -1 inch. The smaller crystals are frequently -more transparent and consequently better -suited for cutting. It is very difficult to -obtain crystals that will allow the cutting -of flawless stones of more than 4 or 5 -carats.</p> -<p>Bluish and colorless celestite of gem -quality and fine crystals have been found -near Lampasas, Lampasas County, and -near Brownwood, Brown County, but -neither of these localities has been very -productive of good gem material.</p> -<p>Celestite geodes have been found in -parts of Coke, Fisher, and Nolan counties, -but these geodes contain little gem material.</p> -<h3 id="c13"><span class="sc">Diamond</span></h3> -<blockquote> -<p><i>Composition</i>: carbon. <i>Crystal system</i>: isometric. -<i>Hardness</i>: 10. <i>Specific gravity</i>: 3.51 to 3.53. <i>Luster</i>: -adamantine to greasy. <i>Color</i>: brown, colorless, -pink, blue, yellow, and various other light -colors; rarely deeply colored; sometimes black. -<i>Cleavage</i>: four directions, octahedral, perfect. -<i>Fracture</i>: conchoidal. <i>Tenacity</i>: brittle. <i>Diaphaneity</i>: -transparent to opaque. <i>Refractive index</i>: -2.42. <i>Dispersion</i>: high.</p> -</blockquote> -<p>There is only one well-authenticated find -of diamond in Texas. A small brownish -diamond was found in 1911 on section 64, -block 44, Foard County (Sterrett, 1912, -pp. 1040-1041). The exact weight of the -stone has not been recorded, but one -authority estimated that it was of sufficient -size and clarity to yield a cut stone of -about one-quarter carat.</p> -<p>The only diamond-bearing rocks known -in the United States are in Pike County, -Arkansas. Although many other diamonds -have been found in the United States, all -were loose in gravels or streams except for -some stones at the Arkansas locality. The -fact that one diamond was found in Foard -County does not mean that the prospects -of finding more diamonds in Texas are -much better there than anywhere else in -the State. It is highly unlikely that more -than a very few diamonds will ever be -found in Texas, and any stones that may -be found in the future are likely to be -widely scattered.</p> -<h3 id="c14"><span class="sc">Epidote</span></h3> -<blockquote> -<p><i>Composition</i>: HCa₂(Al, Fe)₂Si₃O₁₃. <i>Crystal system</i>: -monoclinic. <i>Hardness</i>: 6 to 7. <i>Specific gravity</i>: -3.25 to 3.5. <i>Luster</i>: vitreous. <i>Color</i>: yellowish -green to brownish green and brown. <i>Streak</i>: uncolored -to grayish. <i>Cleavage</i>: two directions. <i>Fracture</i>: -uneven. <i>Tenacity</i>: brittle. <i>Diaphaneity</i>: -transparent to opaque. <i>Refractive index</i>: about -1.72 to 1.77.</p> -</blockquote> -<p>Llano County has furnished some green -and brownish-green epidote that is suitable -<span class="pb" id="Page_20">20</span> -for cutting into cabochons. Most of the -material that approaches gem quality has -come from contact metamorphic zones and -is associated with garnet, quartz, and some -scheelite. Some small cavities in the rocks -contain tiny transparent crystals of gem -quality, but the largest obtainable flawless -faceted stones would probably be less than -15 points.</p> -<p>Faceted stones of epidote are sometimes -known as pistacite owing to their common -pistachio-green color.</p> -<h3 id="c15"><span class="sc">Fluorite</span></h3> -<blockquote> -<p><i>Composition</i>: CaF₂. <i>Crystal system</i>: isometric. -<i>Hardness</i>: 4. <i>Specific gravity</i>: 3.0 to 3.25. <i>Luster</i>: -vitreous. <i>Color</i>: violet, blue, colorless, green, yellow, -brown, rose, and crimson red. <i>Streak</i>: white. -<i>Cleavage</i>: four directions, octahedral, perfect. -<i>Fracture</i>: subconchoidal to splintery. <i>Tenacity</i>: -brittle. <i>Diaphaneity</i>: transparent to subtranslucent. -<i>Refractive index</i>: 1.434.</p> -</blockquote> -<p>Very fine green, transparent fluorite has -been found near Voca, Mason County. The -fluorite occurs as vug fillings in pegmatites, -associated with crystals of pink microcline -and colorless quartz. Most of the vugs have -been completely filled by the fluorite; -therefore, crystals (<a href="#fig15">fig. 15</a>) of the fluorite -are not too common. Masses of fluorite -several pounds in weight, rich green, and -quite transparent have been found near -Voca. Transparent pieces an inch or more -in diameter are common.</p> -<div class="img" id="fig15"> -<img src="images/i07.jpg" alt="" width="400" height="361" /> -<p class="caption"><span class="sc">Fig. 15.</span> Common crystal form of fluorite.</p> -</div> -<p>Fluorite is much too soft for everyday -use in jewelry and because of the low refractive -index does not yield brilliant -faceted stones. The perfect four-directional -cleavage, relative softness, and brittle -tenacity of the mineral make it difficult to -facet. Faceted stones are seldom seen outside -of collections. Cabochons are also -difficult to cut from this material, but the -rich color obtained is ample reward for the -time and care necessary in cutting.</p> -<p>Fluorite occurs at several other localities -in Texas, notably in Hudspeth, Brewster, -Presidio, Llano, and Burnet counties, but -not commonly in gem quality or colors -that warrant its use as gem material.</p> -<h3 id="c16"><span class="sc">Fossil Wood</span></h3> -<p>Wood that is buried in silica-rich sediments -is commonly replaced by quartz, -agate, or opal. The wood structure, including -a large number of the annular rings, -knots, small branches, and bark, may be -preserved. This process of replacement by -silica is believed to take considerable time. -Preservations by other means (<i>see</i> Jet, -<a class="pgref" href="#Page_22">p. 22</a>) are known, but silica replacements -are most commonly used as gem materials.</p> -<p>Fossil wood is often used by lapidaries -as gem material when mineral replacement -preserves the wood structure sufficiently -well and when various impurities color -the replacement material attractively.</p> -<p>Excellent gem-quality fossil wood (<a href="#plateI">Pl. I</a>, B) has been found at a great number of -localities in Texas. Agatized and opalized -wood occurs in great abundance along the -outcrops of Eocene and Oligocene strata -of the Texas Gulf Coastal Plain. Much of -this material is very well suited for cabochons, -bookends, and other lapidary uses. -The preservation is especially good at -numerous localities in Washington, Lee, -Fayette, and Gonzales counties, and the -variety of colors, such as bluish, gray, -brown, red, yellow, and black, makes this -material especially sought after by “rock-hounds.” -Some of the agatized and opalized -wood fluoresces yellow or green under -ultra-violet light. The fossil wood is sometimes -found as stumps, limb sections, or -large trunk fragments, but the great majority -<span class="pb" id="Page_21">21</span> -of the gem material is found as small -broken fragments or stream-rolled cobbles.</p> -<p>Fossil palm wood is by far the most -sought after variety because this material -displays “eyes” and tube-like structures -that yield very attractive cabochons and -cabinet specimens. Texas fossil palm wood -is highly regarded by cutters from all parts -of the country, and this material is thought -by many lapidaries to be some of the finest -gem-quality fossil wood in the United -States.</p> -<p>Gravel pits and river gravels in Live -Oak County have produced very fine agatized -wood. Although the gem material -does not seem to be as abundant in this -area as it is in counties to the northeast, -the vivid colors and excellent preservation -of the fossil wood in Live Oak County -have attracted collectors from all over the -State. The fossil wood usually occurs as -large rounded cobbles in the streams. -Much of this material is quite translucent -when cut and contains various shades of -brown, orange, and red.</p> -<p>The gravels of the Rio Grande have -produced some fossil wood in addition to -the excellent agate that is also found there. -Most of the fossil wood found in these -gravels is very well preserved, but the -colors are commonly dull shades of brown. -Occasional fine red and yellow specimens -have been recovered from the Rio Grande -gravels, but these are rare.</p> -<p>Good agatized wood has been found in -and near Palo Duro Canyon, Armstrong -County, about 50 miles southeast of Amarillo. -Large trunk sections are not uncommon, -but most of the material of cutting -quality is obtained from small fragments. -The Palo Duro Canyon fossil wood greatly -resembles the famous Arizona Petrified -Forest wood but is not nearly as plentiful. -The Palo Duro wood contains yellow, -brown, red, and bluish colors most commonly. -Some of the wood-producing area -is within Palo Duro Canyon State Park -which is, of course, closed to collecting. -The surrounding area has been worked -diligently by local collectors, but new -pieces of wood are exposed after heavy -rains.</p> -<p>Webb and Duval counties have also produced -some good fossil wood specimens.</p> -<h3 id="c17"><span class="sc">Gadolinite</span></h3> -<blockquote> -<p><i>Composition</i>: Be₂FeY₂Si₂O₁₀. (Various other -rare-earth elements may substitute into this mineral -structure.) <i>Crystal system</i>: monoclinic. <i>Hardness</i>: -6.5 to 7.0. <i>Specific gravity</i>: about 4.2. <i>Luster</i>: -vitreous to greasy. <i>Color</i>: black; in thin splinters -dark bottle green. <i>Streak</i>: white to greenish. -<i>Cleavage</i>: none. <i>Fracture</i>: conchoidal to splintery. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: opaque to subtransparent -in thin pieces. <i>Refractive index</i>: variable, -about 1.77 to 1.82.</p> -</blockquote> -<p>Gadolinite as a cut gem is not seen outside -of large collections; however, it can -be faceted into black opaque stones of little -beauty but of great interest to collectors. -The best known locality of this mineral in -the United States is Baringer Hill, Llano -County, Texas. Unfortunately, this locality -was completely flooded by the completion -of Buchanan Dam in 1938. Masses and -rough crystals of gadolinite weighing over -100 pounds were mined from this locality. -The gadolinite occurred in a large, very -coarse-grained pegmatite dike associated -with quartz, microcline, and fluorite, as -well as allanite, fergusonite, nivenite, -cyrtolite, thorogummite, and various other -rare minerals. Some of the minerals in the -dike occurred in very large masses. One -quartz mass over 40 feet in diameter was -noted, and microcline masses up to 30 feet -in diameter were not uncommon. Much of -the gadolinite was used by industrial firms -as a source of thorium compounds, although -some specimen and gem material -found its way into museums and private -collections. Because the locality was -worked mostly from 1910 to about 1925 -and because since 1938 the waters of Lake -Buchanan have completely flooded the -entire area, material from this locality is -now exceedingly difficult to obtain. The -collection of the Smithsonian Institution, -Washington, D.C., contains a cut and polished -gem of Baringer Hill gadolinite -that weighs 8.6 carats. This mineral is -radioactive because of the presence of -<span class="pb" id="Page_22">22</span> -uranium, thorium, and other rare radioactive -elements.</p> -<h3 id="c18"><span class="sc">Garnet</span></h3> -<p>The garnet group of minerals is variable -in composition. Listed below are the pure -members of this group, but garnets found -in nature are usually a mixture of two or -more of these end members.</p> -<dl class="undent"><dt>Aluminum garnet—</dt> -<dd>Grossularite (calcium-aluminum garnet), Ca₃Al₂(SiO₄)₃</dd> -<dd>Pyrope (magnesium-aluminum garnet), Mg₃Al₂(SiO₄)₃</dd> -<dd>Almandite (iron-aluminum garnet), Fe₃Al₂(SiO₄)₃</dd> -<dd>Spessartite (manganese-aluminum garnet), Mn₃Al₂(SiO₄)₃</dd> -<dt>Iron garnet—</dt> -<dd>Andradite (calcium-iron garnet), Ca₃Fe₂(SiO₄)₃; may contain magnesium, titanium, and yttrium</dd> -<dt>Chromium garnet—</dt> -<dd>Uvarovite (calcium-chromium garnet), Ca₃Cr₂(SiO₄)₃</dd></dl> -<p>Since almandite is the only variety of -garnet known to occur commonly in gem -quality in Texas, the following properties -are for almandite except where noted.</p> -<blockquote> -<p><i>Crystal system</i>: isometric (all varieties). <i>Hardness</i>: -about 7.5. <i>Specific gravity</i>: 4.25. <i>Luster</i>: -vitreous to resinous. <i>Color</i>: red, deep red, and -brownish red (other varieties also yellow, white, -orange, pink, black, and green). <i>Streak</i>: white. -<i>Cleavage</i>: none. <i>Fracture</i>: subconchoidal to uneven. -<i>Tenacity</i>: brittle to tough. <i>Diaphaneity</i>: -transparent to subtranslucent. <i>Refractive index</i>: -about 1.83.</p> -</blockquote> -<p>Good crystals of gem-quality almandite -garnet have been found in Llano, Blanco, -Burnet, and Gillespie counties. In southeast -Llano County, northwest Blanco -County, and northeast Gillespie County, -the stones mostly occur in stream gravels -where they have collected after being -weathered out of compact mica schists. -Owing to the fact that most of the garnets -have not been transported very far from -their source, the stones commonly show -good crystal form (<a href="#plateII">Pl. II</a>, A). All of the -garnets from one locality commonly do not -have exactly the same crystal form. The -garnets are mostly widely scattered in the -stream gravels but can be found concentrated -behind rocks and on small gravel -bars.</p> -<p>Many of the crystals are less than one-eighth -inch in diameter; however, good -crystals one-fourth to one-half inch in -diameter are common. Most of the stones -are too fractured or have too many inclusions -to yield gems, but many transparent -stones have been found. The transparent -crystals usually yield flawless deep red -faceted stones of 2 carats or less. Some of -the stones that contain too many inclusions -to facet are cut as cabochons and are then -often known as carbuncle.</p> -<p>Small garnet fragments have been found -in streams and in gneisses and pegmatites -near Castell, Llano County, but they are -not commonly of gem quality.</p> -<p>Occasional small gem-quality garnets -have been found in pegmatites and contact -metamorphic zones in Burnet County. Garnets -have also been found in several other -counties, notably Mason, El Paso, Hudspeth, -and Culberson, but no stones of facet -quality have been reported.</p> -<h3 id="c19"><span class="sc">Jet</span></h3> -<blockquote> -<p><i>Composition</i>: a variety of brown coal or lignite. -<i>Structure</i>: woody. <i>Hardness</i>: 3 to 4. <i>Specific gravity</i>: -about 1.30 to 1.35. <i>Luster</i>: dull. <i>Color</i>: black, -brownish black. <i>Streak</i>: brown to brownish black. -<i>Cleavage</i>: none. <i>Fracture</i>: uneven to smooth. -<i>Tenacity</i>: tough to slightly brittle. <i>Diaphaneity</i>: -opaque. Burns with a sooty yellowish flame.</p> -</blockquote> -<p>Jet is a type of fossil wood in which -there has been sufficient chemical change -to make the wood relatively hard and black -without destroying the woody structure. -The best specimens of jet polish into lustrous -black cabochons.</p> -<p>Jet occurs in Presidio County as compressed -and flattened trunks of trees in a -thin layer of coal and lignite in Cretaceous -strata 100 to 200 feet stratigraphically -below the San Carlos beds.</p> -<p>Specimens of “jet” have been found in -some of the lignitic Tertiary strata of the -Texas Gulf Coastal Plain; however, this -material is mostly soft, brownish, and not -of gem quality.</p> -<div class="pb" id="Page_23">23</div> -<h3 id="c20"><span class="sc">Labradorite</span></h3> -<blockquote> -<p><i>Composition</i>: NaAlSi₃O₈, 50% to 30%; -CaAl₂Si₂O₈, 50% to 70%. <i>Crystal system</i>: triclinic. -<i>Hardness</i>: 6.0 to 6.5. <i>Specific gravity</i>: -about 2.60. <i>Luster</i>: vitreous to sometimes pearly. -<i>Color</i>: straw yellow, white, greenish, gray, reddish, -bluish, and green. Sometimes shows a play -of colors on particular cleavage surfaces. <i>Streak</i>: -uncolored. <i>Cleavage</i>: three directions. <i>Fracture</i>: -uneven to conchoidal. <i>Tenacity</i>: brittle. <i>Diaphaneity</i>: -transparent to translucent. <i>Refractive -index</i>: about 1.56. <i>Dispersion</i>: low.</p> -</blockquote> -<p>Very fine facet-quality labradorite has -been found about 20 miles south of Alpine, -Brewster County. The labradorite occurs -loose in the soil as slightly weathered or -frosted cleavage fragments, commonly -showing one or more crystal faces (<a href="#plateII">Pl. II</a>, B). The pale-yellow or straw-yellow -color of these fragments, as well as their -lack of internal imperfections, makes these -stones excellent gem material. Individual -pieces that exceed three-fourths inch in -their longest dimensions are rare. Cut -stones of more than 5 or 6 carats from this -locality are scarce. The source of this material -is uncertain, but it is probably -weathering out of an underlying igneous -rock.</p> -<h3 id="c21"><span class="sc">Microcline</span></h3> -<blockquote> -<p><i>Composition</i>: KAlSi₃O₈. <i>Crystal system</i>: triclinic. -<i>Hardness</i>: 6.0 to 6.5. <i>Specific gravity</i>: 2.54 -to 2.57. <i>Luster</i>: vitreous to pearly. <i>Color</i>: white, -pale yellow, red, blue green, bluish. <i>Streak</i>: -white. <i>Cleavage</i>: four directions, usually three of -these distinct. <i>Fracture</i>: uneven. <i>Tenacity</i>: brittle -<i>Diaphaneity</i>: transparent to translucent. <i>Refractive -index</i>: about 1.52 to 1.53.</p> -</blockquote> -<p>Very fine crystals of blue microcline -have been found east of Packsaddle Mountain -and near Kingsland in Llano County. -Crystals exceeding 1 foot in length have -been found, although most are only a few -inches long. The color of the microcline is -mostly pale blue, but some crystals are -darker. Microcline crystals associated with -milky or vein quartz, smoky quartz, some -biotite, and rarely cassiterite occur in -pegmatite dikes which vary in size from a -few inches to several feet in thickness. The -color of this microcline is pale in comparison -to microcline from some other localities -in the United States, but the Texas -blue microcline does yield pleasing cabochons. -Perfect crystals of this material -are prized by collectors. Blue or greenish -microcline is often called amazonite or -amazon stone.</p> -<p>Bluish microcline associated with quartz -and topaz has also been reported near -Katemcy, Mason County.</p> -<p>Red microcline is common in several -central Texas counties and is a primary -constituent of many of the igneous rocks -in those counties. Large crystals of perthitic -red microcline occur in pegmatite -dikes of Mason, Llano, Burnet, and Gillespie -counties. Any feldspar quarry or other -pegmatite mining operation in any of these -counties is likely to contain large red microcline -crystals and fragments. Unfortunately, -the good crystals that may have -been present are often shattered by blasting -during quarrying operations.</p> -<p>Feldspar quarries in northeastern Gillespie -County have yielded some good red -cabochon material as well as good crystals. -Here the microcline occurs with milky -and smoky vein quartz, smoky quartz crystals, -clear quartz crystals, greenish muscovite, -and biotite. Many of the older quarries -in Gillespie County have not been -active for some time, and the dumps and -quarry walls have been diligently searched -by collectors.</p> -<div class="img" id="fig16"> -<img src="images/i08.jpg" alt="" width="400" height="295" /> -<p class="caption"><span class="sc">Fig. 16.</span> Crystal faces on microcline specimen shown in <a href="#plateIII">Plate III</a>, A.</p> -</div> -<p>Many of the pegmatite dikes near Lake -Buchanan in Llano and Burnet counties -have produced some good red microcline -specimens and cutting material (<a href="#plateIII">Pl. III</a>, -A, and <a href="#fig16">fig. 16</a>). Many of these crystals are -<span class="pb" id="Page_24">24</span> -more pinkish than those in Gillespie -County, but this is commonly due to the -fact that the crystal faces of the Lake -Buchanan area crystals are somewhat more -weathered than the fresh Gillespie County -crystals.</p> -<p>Numerous other local areas in the counties -mentioned, as well as some localities -in Hudspeth and Culberson counties, have -also produced small amounts of red and -pink microcline of gem quality.</p> -<h3 id="c22"><span class="sc">Obsidian</span></h3> -<blockquote> -<p><i>Composition</i>: volcanic glass. <i>Structure</i>: amorphous. -<i>Hardness</i>: 5.0 to 5.5. <i>Specific gravity</i>: 2.3 -to 2.5. <i>Luster</i>: vitreous. <i>Color</i>: black, dark gray, -reddish, brown, bluish, and greenish. <i>Streak</i>: -white. <i>Cleavage</i>: none. <i>Fracture</i>: conchoidal. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: translucent to -nearly opaque. <i>Refractive index</i>: variable, about -1.45 to 1.53.</p> -</blockquote> -<p>Gem-quality black and dark-gray obsidian -has been found in Presidio County -associated with extrusive igneous rocks. -The obsidian in this area is too opaque to -serve as attractive faceted stones but is -found in pieces of sufficient size and quality -to yield nice cabochons. Some of the -small weathered pieces of this material -resemble tektite in outward appearance; -in fact, the “valverdites” mistaken originally -for tektites are pebbles of weathered -obsidian in terrace gravel of Val Verde -County. Obsidian takes a high polish but -is very sensitive to heat. Stones that are -slightly overheated during grinding or -sanding will quickly shatter.</p> -<p>Obsidian of gem quality has been reported -also in Brewster County.</p> -<h3 id="c23"><span class="sc">Opal</span></h3> -<blockquote> -<p><i>Composition</i>: SiO₂·nH₂O. <i>Structure</i>: amorphous. -<i>Hardness</i>: 5.5 to 6.5. <i>Specific gravity</i>: 1.9 to -2.3. <i>Luster</i>: subvitreous to pearly. <i>Color</i>: white, -bluish, pink, brown, yellow, and gray. <i>Streak</i>: -white. <i>Cleavage</i>: none. <i>Fracture</i>: conchoidal. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: transparent to -nearly opaque. <i>Refractive index</i>: 1.43.</p> -</blockquote> -<p>Opal other than as fossil or opalized -wood (<a href="#Page_20">pp. 20-21</a>) occurs at the following -several localities in Texas.</p> -<p>Approximately 16 miles south of Alpine, -Brewster County, precious opal occurs in -very small seams and as cavity fillings in -very hard pinkish-brown rhyolite. This -opal is milky or bluish and commonly exhibits -small flashes of blue, green, red, and -orange fire. Individual pieces of this opal -are mostly quite small, rarely over one-fourth -inch in diameter, and very difficult -to remove from the tough rhyolite matrix. -Local lapidaries have cut interesting cabochons -from this material in which several -small patches of opal that are close together -in the matrix are included in the -same cabochon.</p> -<p>Small finds of opal associated with rhyolites -and basalts have come from other -localities in west Texas, but the opal -mostly does not display enough play of -colors to warrant its use as gem material.</p> -<p>Near Freer, Duval County, some very -attractive common opal has been found. -The opal is colored various shades of pink, -blue, and yellow and in certain local areas -occurs as fragments that are cemented -together by clear chalcedony. Various -colors are commonly found in the same -piece, and such material yields handsome -cabochons. Although the area has never -been worked commercially, it has been -hunted by collectors and cutters for several -years.</p> -<h3 id="c24"><span class="sc">Pearl</span></h3> -<p>Pearls are the result of the secretion of -calcium carbonate by various shellfish -around sand grains, parasitic organisms, -shell fragments, or other foreign objects -that have in some way entered the body -cavity of the shellfish. Since the shellfish -is unable to expel these irritating particles -or organisms, it deposits successive layers -of calcium carbonate around the foreign -substance to make it smoother and less -irritating. Although pearls are principally -calcium carbonate, they also contain small -amounts of an organic substance, called -conchiolin, and water. Pearls are found in -shellfish that live in either fresh or salt -water. Few pearls are spherical in shape; -most are rounded but somewhat irregular -<span class="pb" id="Page_25">25</span> -and are known as baroque pearls. Good -quality pearls are the only gemstone commonly -sold by the grain, a unit of weight -equal to 0.25 carat or 0.05 gram. The pearl -grain is not the same unit of weight as the -Troy grain.</p> -<p>In Texas, pearls have been found in -fresh-water clams in most of the major -rivers and streams, notably in the Brazos, -Concho, Colorado, Guadalupe, Llano, -Nueces, Sabine, Rio Grande, and Trinity -Rivers. Several Texas lakes have also -yielded pearls, notably Caddo Lake and -other lakes in north-central and northeast -Texas.</p> -<p>Small pearls are frequently found along -the Texas Gulf Coast in edible oysters and -other common shellfish. Fossil pearls have -also been found but because of their -darkened appearance are of value only as -curiosities.</p> -<p>The pearls thus far found in Texas have -been of relatively poor quality and show -little or no iridescence. These pearls have -little value except as curiosities, although -one writer has stated that the discovery -of pearls in the Nueces River led to the -original Spanish settlement of the State -(Baker, 1935, p. 569).</p> -<h3 id="c25"><span class="sc">Quartz</span></h3> -<blockquote> -<p><i>Composition</i>: SiO₂. <i>Crystal system</i>: hexagonal. -<i>Hardness</i>: 7. <i>Specific gravity</i>: 2.65 to 2.66 in crystals. -<i>Luster</i>: vitreous, also waxy, greasy, and dull. -<i>Color</i>: most often colorless, brown, yellow, violet; -sometimes green, red, blue, and black; cryptocrystalline -varieties often variously colored by -impurities. <i>Streak</i>: white. <i>Cleavage</i>: indistinct. -<i>Fracture</i>: conchoidal to splintery. <i>Tenacity</i>: -brittle to tough. <i>Diaphaneity</i>: transparent to -opaque. <i>Refractive index</i>: 1.544 to 1.553.</p> -</blockquote> -<p>The quartz family gemstones can be divided -into two groups for purposes of -description. The first group is the crystalline -varieties, or those quartz varieties that -commonly occur in distinct crystals. The -second group is the cryptocrystalline varieties, -or those quartz varieties that occur -as irregular masses that are composed of -many microscopic crystals. The crystalline -varieties are usually much more transparent -and are most often seen as faceted -stones. The cryptocrystalline varieties vary -from subtransparent to opaque and are -almost always cut as cabochons.</p> -<h3 id="c26"><span class="small">CRYSTALLINE VARIETIES</span></h3> -<p id="c27"><i>Amethyst</i> (violet to purple-colored -quartz).—A northeastern Gillespie County -locality known as Amethyst Hill has -produced quite a number of fine light to -medium violet amethyst crystals which -occur in quartz veins and geodes associated -with serpentine and talc. Many crystals -have been found loose in the soil.</p> -<p>The amethyst tends to be very irregularly -colored in zones parallel to the crystal -faces. In many, the base of the crystal -is colorless or white and only the termination -is violet. Crystals up to 3 inches long -have been found at this locality, but the -average size is much less.</p> -<p>The surface at this locality is almost -entirely depleted of amethyst, with only -an occasional small crystal or fragment -to be seen. However, small excavations are -still sometimes productive.</p> -<p>Good groups of pale amethyst crystals -have been found in quartz veins near the -old town site of Oxford, Llano County. -The occurrence seems to be much the same -as the Amethyst Hill locality. Little exploration -for gemstones has been done in -this area, and future discoveries seem -likely.</p> -<p>Chalcedony geodes lined with amethyst -crystals have been found in Brewster, Presidio, -Culberson, and Hudspeth counties, -but the occurrences are scattered. The -crystals are seldom large enough to yield -gems of more than 3 carats and are mostly -very light colored.</p> -<p>A few pieces of gem-quality amethyst -have been found in Burnet County.</p> -<p id="c28"><i>Citrine</i> (yellow quartz).—Very little -gem-quality citrine has been reported in -Texas. Some small citrine crystals have -been found at Amethyst Hill in northeastern -Gillespie County, but few are of sufficient -size or color to yield good gems.</p> -<p>The writer has seen one citrine crystal -that was found in the gravels of a small -<span class="pb" id="Page_26">26</span> -stream in eastern Llano County near Buchanan -Dam. The crystal weighs about 1 -ounce and is perfectly clear, light golden -yellow, and flawless. However, a further -search of the stream gravels failed to -produce any other citrines.</p> -<p id="c29"><i>Rock crystal</i> (colorless quartz).—Numerous -localities in Texas produce this -colorless variety of quartz, which is the -most common variety of facet quality -quartz and consequently is of little value.</p> -<p>Rock crystal occurs at many localities -in Burnet, Llano, and Mason counties. -The crystals mostly occur in pegmatite -dikes or in stream gravels where they have -been weathered out of their parent rock. -Some fine colorless quartz crystals have -been found near Voca, Mason County, in -weathered pegmatite dikes and also loose -in the sands of nearby streams. Crystals -from this locality are often stained with -reddish iron oxide on their outer surfaces. -Some of the rock crystal found near -Katemcy, Mason County, shows asterism -when cut with the proper orientation. Fine -clear colorless crystals up to 8 inches long -have been found in the pegmatite dikes -near Lake Buchanan in both Llano and -Burnet counties. Several localities near -Enchanted Rock in Llano County have -also produced some good colorless crystals.</p> -<p>Feldspar quarries in large pegmatites -in northeastern Gillespie County have -yielded attractive quartz crystals, some of -which contain smoky phantom crystals and -tourmaline inclusions.</p> -<p>Some pieces of rock crystal enclosing -green, needle-like actinolite crystals have -been found near the Llano-Gillespie-Blanco -County corner. This material is not -suitable for faceted gems but does lend -itself to interesting and attractive cabochons.</p> -<p>Colorless quartz crystals commonly are -found lining small chalcedony geodes in -Brewster, Presidio, Culberson, Hudspeth, -Reeves, and Jeff Davis counties. These -crystals are most commonly less than 1 -inch long but are mostly very clear.</p> -<p>Rock crystal has been found in crevices -of petrified wood in many east and southeast -Texas counties, although the crystals -are mostly quite small.</p> -<p>Many lesser occurrences of rock crystal, -too numerous to mention, are located within -the State.</p> -<p id="c30"><i>Rose quartz</i> (pink quartz).—Rose -quartz occurs at various localities in Burnet, -Llano, Mason, and Gillespie counties, -but the amount of material is mostly small -and the greater part unsuitable for gem -purposes. Some good pink rose quartz occurs -near Town Mountain, Llano County, -but this material does not have flawless -areas large enough to yield faceted stones -of more than a few carats. Rose quartz -is always slightly milky, or cloudy, and -does not cut into brilliant faceted stones. -The Town Mountain rose quartz has been -cut into attractive cabochons.</p> -<p id="c31"><i>Smoky quartz</i> (brown, yellow-brown, -and golden-brown quartz).—Several -Texas localities have produced fine smoky -quartz. Baringer Hill, a noted rare-earth -minerals pegmatite locality in Llano -County, contained some smoky quartz -crystals that were estimated to weigh over -1,000 pounds, and the locality produced -many smaller crystals that were of gem -quality. Baringer Hill was flooded by the -completion of Buchanan Dam in 1938 and -is presently under the waters of Lake -Buchanan. A few fine golden-brown gem-quality -crystals have been found along the -lake shore and in small pegmatites nearby -(<a href="#plateIII">Pl. III</a>, B.).</p> -<p>Feldspar quarries in northeastern Gillespie -County have produced smoky quartz -crystals that exceed 1 foot in length, but -these crystals are mostly flawed, possibly -as a result of blasting, and mostly contain -only small clear areas.</p> -<p>Good color smoky quartz crystals are -found with topaz in the pegmatites and -stream beds in Mason County, near -Streeter, Grit, and Katemcy. These crystals -tend to be lighter colored than those near -Lake Buchanan, but they commonly contain -large flawless areas.</p> -<div class="pb" id="Page_27">27</div> -<h3 id="c32"><span class="small">CRYPTOCRYSTALLINE VARIETIES</span></h3> -<p id="c33"><i>Chalcedony.</i>—When free from impurities -of various oxides and other compounds, -chalcedony has little to render it -pleasing as a gemstone. It is mostly gray, -white, brown, or bluish and commonly has -a waxy luster. Some of the chalcedony -found along the Rio Grande Valley and in -west Texas will take dyes, and local lapidaries -have had some success in dyeing this -material various shades of blue, green, -yellow, and red. When the chalcedony is -naturally colored and variegated, usually -in bands, mossy figures, or dendritic -forms, it is called agate.</p> -<p id="c34"><i>Agate</i> (variegated chalcedony).—The -wide variety of markings and colors available -together with the ease of cutting make -agate a favorite of many lapidaries. Fine -agate has been found at numerous localities -in west and south Texas. Fine plume -agate, famous throughout the United -States, is found south of Alpine. Plume -agate is characterized by dendritic or tree-like -inclusions and is mostly cut into very -handsome cabochons. The agate from -south of Alpine commonly contains black, -red, yellow, or brown plumes within the -same piece. The variety of colors and lack -of porosity of this agate make it highly -desired among lapidaries. The agate occurs -loose on the surface of the ground and -in the soil in small nodules that have a -very rough, brownish surface. These -nodules are mostly less than 3 inches in -diameter, although specimens of gem quality -have been found that exceed 200 -pounds.</p> -<p>Some very fine agate has been found in -the vicinity of Needle Peak, Presidio -County. This material is mostly green moss -agate in clear chalcedony and commonly -contains small yellow “sun-burst” figures. -The contrasting yellow and green design -makes very beautiful cabochons.</p> -<p>Fine agate has been found south of -Marfa, Presidio County. This agate is -mostly clear chalcedony with black, yellow, -or variously colored plumes, moss, or -“bouquet-like” figures.</p> -<p>Numerous other localities in Presidio -and Brewster counties have produced good -agate.</p> -<p>Various amounts of agate, jasper, and -chalcedony occur in the gravels of the Rio -Grande in varying quantities from Big -Bend National Park downstream to -Brownsville. This agate is found both in -the present river gravels and in the older -river gravels that now are located on -nearby hills and slopes up to several miles -north or south of the present Rio Grande. -The greatest concentration of agate and related -gem materials seems to be in the area -between Laredo and Rio Grande City. -Vast quantities of excellent gem material -have been removed from this area for -many years (<a href="#plateIV">Pl. IV</a>). The agate occurs -as rounded, stream-worn cobbles and commonly -has a thin white coating that makes -it difficult to distinguish from the abundant -chert and other rocks. The agate -occurs in cobbles that are mostly 3 to 6 -inches in diameter, but specimens of gem -quality that exceed twice this size are -known. The agate varies greatly in design -and color. Plume, moss, banded, and sagenitic -agate occur in these gravels in a -wide variety of colors. The jasper in the -Rio Grande gravels is yellow, red, green, -or various shades of these and is commonly -suspended as angular fragments in clear -chalcedony.</p> -<p>Good agate has also been found near -Balmorhea in Reeves and Jeff Davis counties -and in smaller amounts at numerous -other west and south Texas localities.</p> -<p id="c35"><i>Agatized wood</i> (<i>see</i> Fossil wood, <a href="#Page_20">pp. 20-21</a>).</p> -<p id="c36"><i>Carnelian</i> (translucent reddish chalcedony).—This -variety of chalcedony in -small quantities has been reported from -near Van Horn, Hudspeth County. Small -pieces of carnelian have been found in the -gravels of the Rio Grande, but finds have -been few and scattered.</p> -<p id="c37"><i>Jasper</i> (impure opaque or subtranslucent -quartz).—Good green, yellow, red, -and brown jasper has been found in the -gravels of the Rio Grande at all of the -<span class="pb" id="Page_28">28</span> -localities that produce agate. The colors -are quite vivid, and the material takes a -fine polish. Some pieces of orbicular jasper -(jasper with circular or eye-like markings) -have been found in this material. -These gravels commonly contain jasper as -fragments that are suspended in clear -chalcedony; this is called brecciated jasper -and yields very handsome cabochons.</p> -<p>Many of the west Texas agate localities -also produce jasper in quantity. Good jasper -has been reported from north of -Brackettville, Kinney County. Jasper is a -minor constituent of the stream gravels in -many parts of the State.</p> -<h3 id="c38"><span class="sc">Sanidine</span></h3> -<blockquote> -<p><i>Composition</i>: KAlSi₃O₈; commonly contains -some sodium. <i>Crystal system</i>: monoclinic. <i>Hardness</i>: -6. <i>Specific gravity</i>: 2.57 to 2.58. <i>Luster</i>: -vitreous to pearly. <i>Color</i>: colorless, white, pale -yellow, and gray. <i>Streak</i>: uncolored. <i>Cleavage</i>: -three directions. <i>Fracture</i>: conchoidal to uneven. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: transparent to -subtranslucent. <i>Refractive index</i>: 1.52 to 1.53.</p> -</blockquote> -<p>Some feldspars, including sanidine, -show a nice blue sheen in reflected light -parallel to certain crystallographic directions. -Stones having this property are -called moonstone. A clear yellowish sanidine -showing an attractive blue sheen has -been found in Brewster, Jeff Davis, and -Presidio counties. The individual pieces -are small, the average size being about -one-eighth inch. The sanidine is found -loose in the soil at some localities where it -has weathered out of rhyolite, and specimens -of the sanidine in the parent rock are -not difficult to obtain. Very small cabochons -can be cut from this material, but -few lapidaries have done so because inexpensive -larger pieces of moonstone can be -obtained easily from foreign sources. However, -the west Texas sanidine does show a -blue sheen when cut and polished.</p> -<h3 id="c39"><span class="sc">Spinel</span></h3> -<blockquote> -<p><i>Composition</i>: MgAl₂O₄ (magnesium may be -replaced in part by ferrous iron or manganese -and the aluminum by ferric iron and chromium). -<i>Crystal system</i>: isometric. <i>Hardness</i>: 8. <i>Specific -gravity</i>: 3.5 to 4.1. <i>Luster</i>: vitreous to sub-metallic. -<i>Color</i>: black, pink, red, blue, green, yellow, -brown, and violet. <i>Streak</i>: white. <i>Cleavage</i>: one -direction, imperfect. <i>Fracture</i>: conchoidal. <i>Tenacity</i>: -brittle. <i>Diaphaneity</i>: transparent to opaque. -<i>Refractive index</i>: variable, approximately 1.72 -to 2.00.</p> -</blockquote> -<p>In many areas of the world, fine quality, -beautifully colored, transparent spinels are -found and used as gems. The only gem-quality -spinel reported thus far in Texas -is black and opaque. Near Eagle Flat in -Hudspeth County, black spinel crystals -have been found associated with augite -and natural glass; these minerals are -weathering out of an intrusive igneous -rock. The spinel crystals have an octahedral -form which is common for this mineral -(<a href="#fig17">fig. 17</a>). Most of the spinels are free -of flaws, but because of their black color -they have little value as gems. The crystals -are found loose in the sand of streams near -the outcrops of the igneous rock or embedded -in the rock. They seldom exceed -half an inch in diameter. These stones are -primarily sought by collectors.</p> -<div class="img" id="fig17"> -<img src="images/i09.jpg" alt="" width="391" height="400" /> -<p class="caption"><span class="sc">Fig. 17.</span> Common crystal form of spinel.</p> -</div> -<h3 id="c40"><span class="sc">Tektite</span> (<span class="sc">Bediasite</span>)</h3> -<blockquote> -<p><i>Composition</i>: A natural glass, approximately -75% SiO₂, 15% Al₂O₃, 4% FeO, also MgO, -Na₂O, K₂O, and traces of other elements. <i>Crystal -structure</i>: amorphous. <i>Hardness</i>: 5 to 6. <i>Specific -gravity</i>: 2.33 to 2.44. <i>Luster</i>: vitreous, often dull -on weathered surfaces. <i>Color</i>: dark brown, greenish -brown, appears black in thick sections. -<i>Streak</i>: uncolored. <i>Cleavage</i>: none. <i>Fracture</i>: -conchoidal. <i>Tenacity</i>: brittle. <i>Diaphaneity</i>: transparent -to subtransparent. <i>Refractive index</i>: 1.488 -to 1.512.</p> -</blockquote> -<p>The average bediasite size is about 1 -inch in diameter, although specimens approximately -<span class="pb" id="Page_29">29</span> -3 inches in diameter are -known. The uncut tektites are very interesting, -showing a variety of shapes and -surface features (<a href="#plateV">Pl. V</a>, A) and many exhibit -contorted flow structure. The surface -of many tektites is grooved or furrowed, -while on others it is smooth or frosted. -The Texas tektites are known as “bediasites,” -after place names in Grimes County -traceable to the Bedias Indians who formerly -lived there.</p> -<p>Dark brown and greenish-brown tektites -have been found in Texas in gravels at -scattered localities in Walker, Grimes, -Brazos, Burleson, Lee, Fayette, Gonzales, -Lavaca, and DeWitt counties. Outside of -Texas the only other authenticated tektite -localities in the United States at the present -time are in Dodge and Irwin counties, -Georgia. A fragment of a similar tektite -has recently been reported from near -Martha’s Vineyard, Massachusetts. The -tektites reported from Oklahoma are now -known to be pebbles of obsidian.</p> -<p>Although tektites have little value or -beauty as gemstones, they have been cut -by lapidaries as both faceted and cabochon -stones. Tektites take a high polish but are -mostly so dark in color that they appear -black.</p> -<p>The origin of tektites is of great scientific -interest and is currently the subject -of much debate. Some scientists believe -that tektites are of meteoritic origin, while -others believe that tektites were formed -by various terrestrial processes. Since no -one has actually observed a tektite to fall -or form, and many of the theories of origin -are difficult to prove without direct observation, -the origin of tektites is likely to -remain in controversy for some time.</p> -<h3 id="c41"><span class="sc">Topaz</span></h3> -<blockquote> -<p><i>Composition</i>: Al₂(F, OH)₂SiO₄. <i>Crystal system</i>: -orthorhombic. <i>Hardness</i>: 8. <i>Specific gravity</i>: 3.4 -to 3.6. <i>Luster</i>: vitreous. <i>Color</i>: pale blue, sky blue, -greenish, white, wine yellow, straw yellow, grayish, -pink, reddish, and orange. <i>Streak</i>: uncolored. -<i>Cleavage</i>: one direction, basal, highly perfect. -<i>Fracture</i>: conchoidal to uneven. <i>Tenacity</i>: brittle. -<i>Diaphaneity</i>: transparent to subtranslucent. -<i>Refractive index</i>: about 1.60 to 1.63. <i>Dispersion</i>: -moderate.</p> -</blockquote> -<p>Various yellow and smoky colored -quartz gems are offered for sale as “Spanish -Topaz,” “Smoky Topaz,” “Madeira -Topaz,” and “Topaz Quartz.” These names -are entirely misleading and should be -dropped from usage.</p> -<p>Fine gem-quality white, pale-blue, and -sky-blue topaz has been found near -Streeter, Grit, and Katemcy, Mason -County. This Texas gem material compares -favorably in color, size, and clarity with -topaz found anywhere in the United States. -Fine crystals of topaz (<a href="#plateV">Pl. V</a>, B, and <a href="#fig18">fig. 18</a>) occasionally are found in pegmatite -dikes associated with quartz, black tourmaline, -cassiterite, and pink microcline. -Many of the gem-bearing pegmatites have -been eroded away, leaving the topaz concentrated -in the stream beds. The stones -mostly occur as frosted, stream-worn pebbles -(<a href="#plateVI">Pl. VI</a>, A) in the numerous small -creeks in the area. The topaz is heavier -than the quartz and microcline that compose -the stream gravel and is commonly -found immediately on top of the granite -bed-rock in the bottom of the stream bed. -The stones tend to lodge behind boulders -or small dikes cutting across the stream.</p> -<div class="img" id="fig18"> -<img src="images/i09a.jpg" alt="" width="263" height="400" /> -<p class="caption"><span class="sc">Fig. 18.</span> Crystal faces on topaz crystal shown in <a href="#plateV">Plate V</a>, B. This crystal habit is typical of the topaz from Mason County.</p> -</div> -<p>The white or colorless stones are by far -the most common, outnumbering the -bluish stones about ten to one. The color -of the blue stones tends to be irregularly -distributed in zones parallel to the crystal -<span class="pb" id="Page_30">30</span> -faces. Topaz that is colored in this manner -should be cut with the best blue color near -the bottom or culet of the gem (<a href="#fig19">fig. 19</a>). -If done correctly, this will give the entire -gemstone the desirable blue color.</p> -<div class="img" id="fig19"> -<img src="images/i10.jpg" alt="" width="400" height="161" /> -<p class="caption"><span class="sc">Fig. 19.</span> Cross section showing the proper orientation of dark-color zone in a gem cut from an irregularly colored stone.</p> -</div> -<dl class="undent pcap"><dt>COLORLESS</dt> -<dt>BLUE</dt></dl> -<p>The colorless stones can be turned pale -yellow, yellowish brown, or straw yellow -by exposure to X-ray radiation, and some -of the bluish stones will fluoresce faintly -yellowish under ultra-violet light.</p> -<p>The largest gem-quality topaz crystal -yet found in North America has come from -Mason County. It is a pale-blue crystal -weighing 1,296 grams, now in the collection -of the U.S. National Museum. Several -other large pieces, some weighing over a -pound, have been found. One large crystal, -exact weight unknown, was found near -Katemcy. Several gem cutters have estimated -that this stone could easily yield a -single, flawless pale-blue gem of about 500 -carats. Many large gems have been cut -from topaz found in this area, including -at least one stone of over 300 carats.</p> -<p>One obstacle in the cutting of topaz is -its perfect basal cleavage. The gemstone -should be oriented so that no facet of the -stone will be parallel to or within less than -about 5 degrees of the cleavage direction, -or the facet may be very difficult or impossible -to polish.</p> -<p>It is difficult to estimate the productivity -of this area since its discovery in the early -1900’s. Few systematic attempts have been -made to exploit the deposits, and a great -amount of the topaz thus far recovered has -been found by private collectors. The -Mason County topaz deposits are still very -productive, and additional exploration -may uncover even more gem-producing -areas.</p> -<p>Topaz has also been found in stream -gravels or pegmatites in Burnet, Llano, -Gillespie, and El Paso counties but very -rarely in gem quality.</p> -<h3 id="c42"><span class="sc">Tourmaline</span></h3> -<blockquote> -<p><i>Composition</i>: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen -often replaced by iron, magnesium, calcium, or -fluorine. <i>Crystal system</i>: hexagonal. <i>Hardness</i>: 7 -to 7.5. <i>Specific gravity</i>: 2.98 to 3.20. <i>Luster</i>: vitreous -to resinous. <i>Color</i>: black, brownish black, -brown, blue, green, red, pink, yellow, and gray. -<i>Streak</i>: uncolored. <i>Cleavage</i>: two directions, very -imperfect. <i>Fracture</i>: subconchoidal to uneven. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: transparent to -opaque. <i>Refractive index</i>: about 1.62 to 1.64.</p> -</blockquote> -<p>Black tourmaline is schorl; -brown tourmaline, dravite.</p> -<p>Good crystals of black and dark brown -tourmaline occur at Town Mountain near -Llano, Llano County. The tourmaline -crystals average about 1 inch in length, -do not commonly exceed 2 inches, and -are associated with white vein quartz. -The quartz completely encloses the tourmaline, -but the crystals can be broken free -or the quartz can be trimmed away with -the use of a diamond saw. The latter procedure -is recommended whenever possible, -for it is very easy to shatter the tourmaline -crystals while trying to remove them from -the quartz by other means. Many of the -crystals are completely unsuitable for cutting, -being too brittle or too badly cracked -and flawed. However, some small crystals -have been found that are of sufficient quality -and size to yield flawless stones of a -few carats. Few of these stones have been -cut since the tourmaline is so dark that it -appears opaque, and few persons find a -gem of this nature attractive.</p> -<p>Good black and dark brown crystals of -tourmaline associated with andalusite and -graphite occur in the Packsaddle schist -(Precambrian) near Sunrise Beach, Llano -County (<a href="#plateVI">Pl. VI</a>, B, and <a href="#fig20">fig. 20</a>). Although -generally smaller in diameter than the -crystals found at Town Mountain, they -commonly exceed 3 inches in length, although -the average size is a little over 1 -inch. Many of these crystals are suitable -<span class="pb" id="Page_31">31</span> -for cutting into opaque or nearly opaque -stones of about 5 or 6 carats.</p> -<p>Black tourmaline has also been found -in Hudspeth and Culberson counties but -not of sufficient quality to be used as a -gemstone.</p> -<div class="img" id="fig20"> -<img src="images/i10a.jpg" alt="" width="243" height="501" /> -<p class="caption"><span class="sc">Fig. 20.</span> Common crystal form of Llano County tourmaline.</p> -</div> -<h3 id="c43"><span class="sc">Turquoise</span></h3> -<blockquote> -<p><i>Composition</i>: hydrous phosphate of aluminum -and copper. <i>Crystal system</i>: triclinic. <i>Hardness</i>: -5 to 6. <i>Specific gravity</i>: variable, 2.6 to about 2.8. -<i>Luster</i>: dull, sometimes waxy. <i>Color</i>: sky blue to -greenish blue. <i>Streak</i>: white to greenish. -<i>Cleavage</i>: none in massive material, two directions -in crystals. <i>Fracture</i>: conchoidal to subconchoidal. -<i>Tenacity</i>: brittle. <i>Diaphaneity</i>: subtranslucent -to opaque. <i>Refractive index</i>: 1.61 to -1.65.</p> -</blockquote> -<p>Turquoise of good sky-blue to greenish-blue -color has been found a few miles -southwest of Van Horn, Culberson County. -Several shallow pits were dug at this locality -about 1910; however, the amount -of turquoise produced was small. The main -occurrence of the turquoise was in seams -about 1 millimeter thick along joints in the -fine-grained rocks of this area. Persons -who have visited Culberson County more -recently report that even minute traces of -the turquoise are now difficult to find at -the old prospect pits. However, further -prospecting in the area might yield some -additional localities.</p> -<p>Small amounts of turquoise have been -reported near El Paso, El Paso County, -and also in volcanic rocks near the Jeff -Davis-Brewster County line, north of Alpine.</p> -<p>A small amount of turquoise has been -mined from several localities a few miles -northwest of Sierra Blanca in the Sierra -Blanca Mountains of Hudspeth County.</p> -<div class="pb" id="Page_32">32</div> -<h2 id="c44">GLOSSARY</h2> -<dl class="undent"><dt><b>Amorphous</b>—without definite molecular structure; not crystalline.</dt> -<dt><b>Baroque stone</b>—an irregularly shaped, polished stone; usually applied to tumbled stones.</dt> -<dt><b>Baroque pearl</b>—an irregularly shaped pearl.</dt> -<dt><b>Brilliancy</b>—reflecting much light; having brightness.</dt> -<dt><b>Brilliant cut</b>—a mode of arrangement of facets commonly used on round or oval stones. The standard American brilliant cut has 57 or 58 facets. Most diamonds of 5 or less carats are cut in this manner.</dt> -<dt><b>Cabochon</b>—a stone cut with a flat or convex upper surface; sometimes faceted in part. Opal, star sapphire, and agate are stones that are frequently cut in this style (<a href="#fig2">fig. 2</a>).</dt> -<dt><b>Cambrian</b>—a division of geologic time, estimated to be the time from 550 to 440 million years ago; the oldest time division of the Paleozoic era.</dt> -<dt><b>Carat</b>—a unit of weight equal to ⅕ of a gram or 0.2 gram. One ounce avoirdupois is equal to 141.75 carats.</dt> -<dt><b>Cleavage</b>—the tendency of certain minerals to split in particular directions yielding relatively smooth plane surfaces.</dt> -<dt><b>Conchiolin</b>—an organic albuminoid substance found in pearls.</dt> -<dt><b>Conchoidal</b>—a type of fracture having curved concavities or the approximate shape of one-half of a bivalve shell. Glass has excellent conchoidal fracture.</dt> -<dt><b>Cretaceous</b>—a division of geologic time, estimated to be the time from 135 to 60 million years ago; youngest division of the Mesozoic era.</dt> -<dt><b>Crown</b>—that portion of a faceted gem above the girdle; the upper portion of a facet-cut gem (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Cryptocrystalline</b>—composed of very fine or microscopic crystals.</dt> -<dt><b>Crystal</b>—the regular polyhedral form, bounded by plane surfaces, that is assumed by a mineral under suitable conditions. Crystals have definite external symmetry and internal molecular order.</dt> -<dt><b>Crystalline</b>—possessing definite internal molecular order; not amorphous.</dt> -<dt><b>Cubic</b>—in the general shape of a cube. The isometric crystal system is often called the cubic system.</dt> -<dt><b>Culet</b>—the very bottom portion of a faceted gem; the point or line formed by the intersection of the lowest pavilion facets (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Dendritic</b>—branching or tree-like in form.</dt> -<dt><b>Diaphaneity</b>—relative transparency. The diaphaneity of a mineral is described as transparent, translucent, opaque, etc.</dt> -<dt><b>Dike</b>—a tabular rock body, usually igneous in origin, which cuts across the surrounding rock strata.</dt> -<dt><b>Dispersion</b>—a measure of the ability of gemstones to separate complex or white light into its component colors; often illustrated with a prism. Gemstones that are capable of separating colors of light widely are said to have high dispersion; gemstones not so capable of separating white light into colors are said to have low dispersion.</dt> -<dt><b>Dopping</b>—the act of cementing a gemstone, either rough or partly finished, to a dop-stick.</dt> -<dt><b>Dop-stick</b>—the wooden stick or cylindrical piece of metal to which a gemstone is cemented to facilitate handling during cutting and polishing.</dt> -<dt><b>Dop-wax</b>—the agent or cement used to secure a gemstone to a dop-stick.</dt> -<dt><b>Emerald cut</b>—a rectangular or square faceted stone with beveled corners whose surfaces are covered with several series of rectangular facets.</dt> -<dt><b>Eocene</b>—a division of geologic time, estimated to be the time from 50 to 40 million years ago; one of the older divisions of the Cenozoic era.</dt> -<dt><b>Extrusive rock</b>—igneous rock that has been extruded or forced out onto the earth’s surface.</dt> -<dt><b>Facet</b>—a single plane polished surface on a faceted gem.</dt> -<dt><b>Facet head</b>—a device used in the cutting and polishing of faceted gems; used to control the placement of facets and their relative angles (<a href="#fig7">fig. 7</a>).</dt> -<dt><b>Facet table</b>—the equipment used in the cutting and polishing of faceted gems and the table on which most of the equipment is mounted (<a href="#fig7">fig. 7</a>).</dt> -<dt><b>Feldspar</b>—a group of closely related silicate minerals including orthoclase, microcline, sanidine, plagioclase, labradorite, and others.</dt> -<dt><b>Fire</b>—the reflections of variously colored light from a precious opal; also the different colors of light reflected from a faceted gem owing to the dispersion of the mineral.</dt> -<dt><b>Fracture</b>—the texture of a freshly broken surface other than a cleavage surface, described as conchoidal, even, splintery, etc.</dt> -<dt><b>Gem</b>—a cut and polished gemstone.</dt> -<dt><b>Gemology</b>—the science dealing with the study of gemstones.</dt> -<dt><b>Gemstone</b>—a mineral suitable for cutting into a gem; the term gemstones is frequently used collectively to include both cut and polished stones and rough stones.</dt> -<dt><b>Geode</b>—a rounded or spherical rock cavity; commonly lined with crystals.</dt> -<dt><b>Girdle</b>—the portion of a faceted gem separating the crown from the pavilion; the girdle may or may not be polished and usually contains about 2 percent of the total depth of the gem (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Gneiss</b>—a coarse-grained metamorphic rock having segregations of granular and platy minerals that give it a more or less banded appearance without well-developed schistosity.</dt> -<dt><b>Grain</b> (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat; not the same as the Troy grain.</dt> -<dt class="pb" id="Page_33">33</dt> -<dt><b>Granite</b>—a granular igneous rock composed mostly of quartz, feldspar, and commonly mica and/or hornblende.</dt> -<dt><b>Hexagonal</b>—having six angles and six sides; a crystal system in which the crystal faces are referred to four intersecting axes; three of these axes are equal, lie in the same plane, and intersect at angles of 60 degrees; the fourth axis is perpendicular to the other three.</dt> -<dt><b>Igneous rock</b>—rock formed by solidification from a hot melt.</dt> -<dt><b>Index of refraction</b>—a measure of the relative ability of a gemstone to “bend” incident light rays; sine of the angle of incidence of a light ray divided by the sine of the angle of refraction.</dt> -<dt><b>Intrusive rock</b>—rock that has been pushed (usually in a molten state) among pre-existing rock strata, commonly along faults or fissures. Intrusive rocks do not reach the earth’s surface but are commonly exposed at the surface by later erosion.</dt> -<dt><b>Isometric</b>—a crystal system in which the crystal faces are referred to three equal intersecting axes at right angles to each other.</dt> -<dt><b>Lap</b>—a disc-shaped piece of metal or other material which is impregnated with diamond dust, or some other cutting or polishing agent, that is revolved while the gemstone is worked against it.</dt> -<dt><b>Lap plate</b>—a metal plate to which a cutting or polishing lap is attached, usually by means of a threaded bolt and wing nut. The lap plate is attached to the shaft which is turned by the motor under the facet table.</dt> -<dt><b>Lapidary</b>—one who practices the lapidary arts; a gem cutter.</dt> -<dt><b>Limestone</b>—a sedimentary rock composed mostly of calcium carbonate.</dt> -<dt><b>Luster</b>—the appearance of the freshly broken or unweathered surface of a mineral in reflected light (<a class="pgref" href="#Page_5">p. 5</a>).</dt> -<dt><b>Main facet</b>—as applied to the standard American brilliant cut, one of the first eight facets cut on either the crown or pavilion of a gem (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Matrix</b>—the material in which a specific mineral is embedded; also the rock to which one end of a crystal is attached.</dt> -<dt><b>Metamorphic rock</b>—rock that has been changed from its original state by heat, pressure, chemical action, or some combination of these factors.</dt> -<dt><b>Millimeter</b>—¹/₁₀ centimeter; approximately ¹/₂₅ inch.</dt> -<dt><b>Mineralogy</b>—the science concerned with the study of minerals, including their occurrence, composition, forms, properties, and structure.</dt> -<dt><b>Monoclinic</b>—a crystal system in which the crystal faces are described in relation to three intersecting unequal axes, two of which are at right angles and the third inclined.</dt> -<dt><b>Oligocene</b>—a division of geologic time, estimated to be the time from 40 to 28 million years ago; part of the Cenozoic era.</dt> -<dt><b>Opaque</b>—does not transmit light.</dt> -<dt><b>Orbicular</b>—containing orbs or spherical or eye-like markings or structures.</dt> -<dt><b>Orthorhombic</b>—a crystal system in which crystal faces are referred to three unequal intersecting axes at right angles.</dt> -<dt><b>Pavilion</b>—the portion of a faceted gem below the girdle (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Pegmatite</b>—a body of coarse-grained intrusive igneous rock, commonly lens or dike shaped.</dt> -<dt><b>Perthitic</b>—a plaid-patterned structure resulting from intermixture of soda- and potash-rich feldspars.</dt> -<dt><b>Phantom crystal</b>—a crystal outline seen within another crystal, mostly due to entrapping of inclusions during the crystal’s growth.</dt> -<dt><b>Pleochroism</b>—the property of transmitting different colors of light in different crystallographic directions.</dt> -<dt><b>Point</b>—a unit of weight equal to ¹/₁₀₀ (0.01) carat.</dt> -<dt><b>Porous</b>—containing pores or void spaces.</dt> -<dt><b>Precambrian</b>—a division of geologic time, estimated to be all of geologic time prior to 550 million years ago; the time before the Paleozoic era.</dt> -<dt><b>Preform</b>—a gemstone that has been ground to a rough outline of the finished shape of a gem.</dt> -<dt><b>Rhyolite</b>—a fine-grained extrusive or shallow intrusive igneous rock of approximately the same composition as granite.</dt> -<dt><b>Rough</b>—uncut, not worked by a lapidary, not cut and polished.</dt> -<dt><b>Schist</b>—a metamorphic rock that contains an abundance of oriented platy minerals that enable the rock to be split with relative ease parallel to the flat surfaces of the platy minerals.</dt> -<dt><b>Silicified</b>—replaced by or containing a large amount of quartz or silica.</dt> -<dt><b>Skill facet</b>—a term often used for the pavilion girdle facets of the standard American brilliant cut (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Specific gravity</b>—the weight in air divided by the loss of weight in water at a given temperature, or the weight of an object in air divided by the weight of an equal volume of water; also called relative density; the most commonly used standard temperature for this measurement is 4° C. or 39.2° F.</dt> -<dt><b>Star facet</b>—one of the eight facets surrounding the table facet of a standard American brilliant cut (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Step cut</b>—a mode of faceting in which the surface of the gem is covered by a series of square or rectangular facets; stones thusly cut are usually square, rectangular, or irregular with straight sides in outline.</dt> -<dt><b>Streak</b>—the color of a mineral when finely powdered; usually determined by rubbing the mineral against a piece of unglazed porcelain.</dt> -<dt class="pb" id="Page_34">34</dt> -<dt><b>Symmetry</b>—the number, location, and balanced arrangement of crystal faces in reference to the crystallographic axes or other crystallographic planes or directions.</dt> -<dt><b>Synthetic gem</b>—a gemstone manufactured by man that has approximately the same chemical composition and properties as a natural gemstone.</dt> -<dt><b>Table facet</b>—the large horizontal facet found on the crown of many gems, often called simply the table (<a href="#fig6">fig. 6</a>).</dt> -<dt><b>Tenacity</b>—the resistance of minerals to breakage, described by such terms as malleable, ductile, sectile, and brittle (<a class="pgref" href="#Page_6">p. 6</a>).</dt> -<dt><b>Termination</b>—the end of a crystal that is completely enclosed by crystal faces, the crystal end that is not attached to the matrix.</dt> -<dt><b>Tertiary</b>—a division of geologic time, estimated to be the time from 60 to 1 million years ago; the Tertiary includes the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs (from oldest to youngest).</dt> -<dt><b>Tetragonal</b>—having four angles; a crystal system in which the crystal faces are referred to three axes at right angles to each other, two of which are equal and the third longer or shorter.</dt> -<dt><b>Translucent</b>—allowing the passage of light but diffusing it sufficiently so that objects on the other side cannot be clearly distinguished.</dt> -<dt><b>Transparent</b>—clear, allowing free passage of light so that objects on the other side can be readily distinguished; opposite of opaque.</dt> -<dt><b>Triclinic</b>—a crystal system in which the crystal faces are referred to three unequal axes, none of which are at right angles.</dt> -<dt><b>Tumbling</b>—a process of polishing irregularly shaped gemstones (<a class="pgref" href="#Page_17">p. 17</a>).</dt> -<dt><b>Vein</b>—a tabular, irregular, or twisting mineral deposit that is thin in relation to its length and breadth, usually the result of solution or hydrothermal activity.</dt> -<dt><b>Vitreous</b>—having luster, general appearance, or physical properties similar to glass.</dt> -<dt><b>Vug</b>—an unfilled rock cavity, commonly lined with crystals; may later become filled by minerals owing to solution or hydrothermal activity.</dt></dl> -<h2 id="c45">SELECTED REFERENCES</h2> -<p class="revint"><span class="sc">Anderson, B. W.</span> (1948) Gem testing: Emerson, -New York.</p> -<p class="revint"><span class="sc">Baker, C. L.</span> (1935) Metallic and non-metallic -minerals and ores (precious stones), <i>in</i> The -geology of Texas, Vol. II, Structural and economic -geology: Univ. Texas Bull. 3401, Jan. 1, -1934, pp. 568-569.</p> -<p class="revint"><span class="sc">Barnes, V. E.</span> (1940) North American tektites: -Univ. Texas Pub. 3945, Dec. 1, 1939, pp. 477-582.</p> -<p class="revint"><span class="sc">Dake, H. C.</span>, <span class="sc">Fleener, F. L.</span>, and <span class="sc">Wilson, B. H.</span> -(1938) Quartz family minerals: Whittlesey -House, McGraw-Hill Book Company, Inc., New -York.</p> -<p class="revint"><span class="sc">Ford, W. E.</span> (1932) A textbook of mineralogy -(4th ed.): John Wiley and Sons, Inc., New -York.</p> -<p class="revint"><span class="sc">Kraus, E. H.</span>, and <span class="sc">Slawson, C. B.</span> (1947) Gems -and gem materials (5th ed.): McGraw-Hill -Book Company, Inc., New York.</p> -<p class="revint"><span class="sc">Kunz, G. F.</span> (1892) Gems and precious stones of -North America (2d ed.): Scientific Publishing -Company, New York.</p> -<p class="revint"><span class="sc">Pough, F. H.</span> (1953) A field guide to rocks and -minerals: Houghton Mifflin Company, Boston.</p> -<p class="revint"><span class="sc">Simpson, B. W.</span> (1958) Gem trails of Texas: -Granbury, Texas.</p> -<p class="revint"><span class="sc">Sinkankas, John</span> (1955) Gem cutting: D. Van -Nostrand Company, Inc., Princeton, New -Jersey.</p> -<p class="revint">—— (1959) Gemstones of North America: -D. Van Nostrand Company, Inc., Princeton, -New Jersey.</p> -<p class="revint"><span class="sc">Smith, G. F. H.</span> (1958) Gemstones (13th ed.), -revised by F. C. Phillips: Methuen and Company, -Ltd., London.</p> -<p class="revint"><span class="sc">Sperisen, F. J.</span> (1950) The art of the lapidary: -The Bruce Publishing Company, Milwaukee, -Wisconsin.</p> -<p class="revint"><span class="sc">Sterrett, D. B.</span> (1913) Gems and precious -stones, <i>in</i> Mineral resources of the United -States, Calendar Year 1912, Part II, Non-metals: -U. S. Geol. Survey, pp. 1023-1060.</p> -<div class="pb" id="Page_35">35</div> -<h3><a id="plateI">Plate I</a></h3> -<div class="img" id="fig21"> -<img src="images/i11.jpg" alt="" width="800" height="626" /> -<p class="caption">A -<br />Gem-quality celestite crystals from Travis County, Texas. Twice natural size. Lower portion of the crystals is colorless; the tips are dark blue.</p> -</div> -<div class="img" id="fig22"> -<img src="images/i11a.jpg" alt="" width="800" height="659" /> -<p class="caption">B -<br />Opalized wood from the Texas Gulf Coastal Plain. Specimen at left is rich brown and tan; specimen at right is fossil palm wood and is black, reddish brown, and white. One-third natural size.</p> -</div> -<div class="pb" id="Page_36">36</div> -<h3><a id="plateII">Plate II</a></h3> -<div class="img" id="fig23"> -<img src="images/i12.jpg" alt="" width="800" height="657" /> -<p class="caption">A -<br />Gem-quality garnet crystals and faceted gem from Gillespie County, Texas. Natural size.</p> -</div> -<div class="img" id="fig24"> -<img src="images/i12a.jpg" alt="" width="800" height="682" /> -<p class="caption">B -<br />Labradorite from Brewster County, Texas. Both stones are pale yellow. One and a half times natural size.</p> -</div> -<div class="pb" id="Page_37">37</div> -<h3><a id="plateIII">Plate III</a></h3> -<div class="img" id="fig25"> -<img src="images/i12c.jpg" alt="" width="800" height="727" /> -<p class="caption">A -<br />Pink microcline crystal from Burnet County, Texas.</p> -</div> -<div class="img" id="fig26"> -<img src="images/i12d.jpg" alt="" width="800" height="547" /> -<p class="caption">B -<br />Smoky quartz from Burnet County, Texas. Natural size. Colorless crystal at center back is included for color comparison.</p> -</div> -<div class="pb" id="Page_38">38</div> -<h3><a id="plateIV">Plate IV</a></h3> -<div class="img" id="fig27"> -<img src="images/i13.jpg" alt="" width="700" height="570" /> -<p class="caption">Polished agate from gravels of the Rio Grande near Zapata, Zapata County, Texas. Bands are blue and gray; other inclusions are brown, yellow, and reddish. One and a half times natural size.</p> -</div> -<div class="pb" id="Page_39">39</div> -<h3><a id="plateV">Plate V</a></h3> -<div class="img" id="fig28"> -<img src="images/i13b.jpg" alt="" width="800" height="289" /> -<p class="caption">A -<br />Texas tektites (bediasites) showing variety of surface features. Natural size.</p> -</div> -<div class="img" id="fig29"> -<img src="images/i13c.jpg" alt="" width="508" height="800" /> -<p class="caption">B -<br />Topaz crystal from a pegmatite dike near Streeter, Mason County, Texas. Natural size. Measurements: 1½ by 1⅝ by 3 inches; weight: 194 grams (970 carats); pale blue; mostly gem quality.</p> -</div> -<div class="pb" id="Page_40">40</div> -<h3><a id="plateVI">Plate VI</a></h3> -<div class="img" id="fig30"> -<img src="images/i14.jpg" alt="" width="800" height="546" /> -<p class="caption">A -<br />Topaz from stream gravels near Streeter, Mason County, Texas. Natural size. Left to right: colorless worn pebble; emerald-cut pale-blue topaz, weight 10 carats; pale-blue worn pebble, weight 205 carats; step out sky-blue topaz, weight 13 carats; pale-blue worn pebble.</p> -</div> -<div class="img" id="fig31"> -<img src="images/i14a.jpg" alt="" width="800" height="708" /> -<p class="caption">B -<br />Tourmaline crystals in schist from Llano County, Texas.</p> -</div> -<div class="pb" id="Page_41">41</div> -<h2 id="c46">Index</h2> -<p class="center"><span class="ab">A</span> <a class="ab" href="#index_B">B</a> <span class="ab">C</span> <a class="ab" href="#index_D">D</a> <a class="ab" href="#index_E">E</a> <span class="ab">F</span> <span class="ab">G</span> <span class="ab">H</span> <span class="ab">I</span> <span class="ab">J</span> <a class="ab" href="#index_K">K</a> <span class="ab">L</span> <a class="ab" href="#index_M">M</a> <span class="ab">N</span> <span class="ab">O</span> <a class="ab" href="#index_P">P</a> <span class="ab">Q</span> <span class="ab">R</span> <a class="ab" href="#index_S">S</a> <span class="ab">T</span> <a class="ab" href="#index_U">U</a> <span class="ab">V</span> <a class="ab" href="#index_W">W</a> <span class="ab">X</span> <span class="ab">Y</span> <a class="ab" href="#index_Z">Z</a></p> -<dl class="index"> -<dt class="center" id="index_A"><b>A</b></dt> -<dt>actinolite: <a class="pgref" href="#Page_26">26</a></dt> -<dt>agate: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_28">28</a>, <a class="pgref" href="#Page_38">38</a></dt> -<dt>agatized wood: <a class="pgref" href="#Page_27">27</a></dt> -<dt>allanite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>almandite: <a class="pgref" href="#Page_22">22</a></dt> -<dt>amazonite: <a class="pgref" href="#Page_23">23</a></dt> -<dt>amazon stone: <a class="pgref" href="#Page_23">23</a></dt> -<dt>amber: <a class="pgref" href="#Page_18">18</a></dt> -<dt>amethyst: <a class="pgref" href="#Page_25">25</a></dt> -<dt>Amethyst Hill: <a class="pgref" href="#Page_25">25</a></dt> -<dt>amorphous gemstones: <a class="pgref" href="#Page_9">9</a></dt> -<dt>andalusite: <a class="pgref" href="#Page_30">30</a></dt> -<dt>Arkansas: <a class="pgref" href="#Page_19">19</a></dt> -<dt>Armstrong County: <a class="pgref" href="#Page_21">21</a></dt> -<dt>augite: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_28">28</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_B"><b>B</b></dt> -<dt>Baringer Hill, Llano County: <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_26">26</a></dt> -<dt>baroque pearls and/or stones: <a class="pgref" href="#Page_17">17</a>, <a class="pgref" href="#Page_25">25</a></dt> -<dt>bediasite (tektite): <a class="pgref" href="#Page_28">28</a>-29, <a class="pgref" href="#Page_39">39</a></dt> -<dt>beryl: <a class="pgref" href="#Page_18">18</a></dt> -<dt>Big Bend National Park: <a class="pgref" href="#Page_27">27</a></dt> -<dt>biotite: <a class="pgref" href="#Page_23">23</a></dt> -<dt>Blanco County: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_22">22</a></dt> -<dt>Brazos County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Brazos River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>Brewster County: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_24">24</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_27">27</a>, <a class="pgref" href="#Page_28">28</a>, <a class="pgref" href="#Page_31">31</a>, <a class="pgref" href="#Page_36">36</a></dt> -<dt>brilliancy: <a class="pgref" href="#Page_5">5</a></dt> -<dt>brilliant cut, standard American: <a class="pgref" href="#Page_13">13</a>, <a class="pgref" href="#Page_15">15</a>, <a class="pgref" href="#Page_16">16</a></dt> -<dt>Brown County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>Burleson County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Burnet County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_30">30</a>, <a class="pgref" href="#Page_37">37</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_C"><b>C</b></dt> -<dt>cabochon gems: <a class="pgref" href="#Page_10">10</a>-12</dt> -<dt>Caddo Lake: <a class="pgref" href="#Page_25">25</a></dt> -<dt>carbuncle: <a class="pgref" href="#Page_22">22</a></dt> -<dt>carnelian: <a class="pgref" href="#Page_27">27</a></dt> -<dt>cassiterite: <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_29">29</a></dt> -<dt>celestite: <a class="pgref" href="#Page_19">19</a>, <a class="pgref" href="#Page_35">35</a></dt> -<dt>chalcedony: <a class="pgref" href="#Page_27">27</a></dt> -<dd>geodes: <a class="pgref" href="#Page_26">26</a></dd> -<dt>chuck: <a class="pgref" href="#Page_15">15</a>, <a class="pgref" href="#Page_17">17</a></dt> -<dt>citrine: <a class="pgref" href="#Page_25">25</a>-26</dt> -<dt>cleavage: <a class="pgref" href="#Page_6">6</a>, <a class="pgref" href="#Page_13">13</a></dt> -<dt>coal: <a class="pgref" href="#Page_22">22</a></dt> -<dt>Coke County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>color: <a class="pgref" href="#Page_5">5</a></dt> -<dt>Colorado River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>Concho River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>crown girdle facets: <a class="pgref" href="#Page_16">16</a>, <a class="pgref" href="#Page_17">17</a></dt> -<dt>crown of gemstone: <a class="pgref" href="#Page_15">15</a>, <a class="pgref" href="#Page_16">16</a></dt> -<dt>crystals: <a class="pgref" href="#Page_7">7</a>-9</dt> -<dt>crystal systems: <a class="pgref" href="#Page_7">7</a></dt> -<dt>crytolite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>Culberson County: <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_31">31</a></dt> -<dt>culet: <a class="pgref" href="#Page_13">13</a></dt> -<dt>cutting and polishing: <a class="pgref" href="#Page_10">10</a>-17</dt> -<dt>cutting lap: <a class="pgref" href="#Page_13">13</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_D"><b>D</b></dt> -<dt>DeWitt County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>diamond: <a class="pgref" href="#Page_19">19</a></dt> -<dd>saw: <a class="pgref" href="#Page_10">10</a>, <a class="pgref" href="#Page_11">11</a></dd> -<dt>diaphaneity: <a class="pgref" href="#Page_5">5</a></dt> -<dt>dispersion: <a class="pgref" href="#Page_6">6</a></dt> -<dt>dopping: <a class="pgref" href="#Page_12">12</a>, <a class="pgref" href="#Page_13">13</a></dt> -<dt>dop-stick: <a class="pgref" href="#Page_12">12</a>, <a class="pgref" href="#Page_15">15</a>, <a class="pgref" href="#Page_17">17</a></dt> -<dt>dop-wax: <a class="pgref" href="#Page_12">12</a>, <a class="pgref" href="#Page_15">15</a>, <a class="pgref" href="#Page_17">17</a></dt> -<dt>dravite: <a class="pgref" href="#Page_30">30</a></dt> -<dt>durability: <a class="pgref" href="#Page_6">6</a></dt> -<dt>Duval County: <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_24">24</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_E"><b>E</b></dt> -<dt>El Paso County: <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_30">30</a>, <a class="pgref" href="#Page_31">31</a></dt> -<dt>emerald cut: <a class="pgref" href="#Page_15">15</a></dt> -<dt>epidote: <a class="pgref" href="#Page_19">19</a>-20</dt> -</dl> -<dl class="index"> -<dt class="center" id="index_F"><b>F</b></dt> -<dt>facet, kinds of: <a class="pgref" href="#Page_13">13</a></dt> -<dd>main: <a class="pgref" href="#Page_16">16</a></dd> -<dd>skill: <a class="pgref" href="#Page_16">16</a></dd> -<dd>table: <a class="pgref" href="#Page_13">13</a>, <a class="pgref" href="#Page_14">14</a></dd> -<dt>faceted gems and/or stones: <a class="pgref" href="#Page_10">10</a>, <a class="pgref" href="#Page_13">13</a>-17</dt> -<dt>Fayette County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_29">29</a></dt> -<dt>fergusonite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>fire: <a class="pgref" href="#Page_5">5</a></dt> -<dt>Fisher County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>fluorite: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_21">21</a></dt> -<dt>Foard County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>fossil wood: <a class="pgref" href="#Page_20">20</a>-21, <a class="pgref" href="#Page_22">22</a></dt> -<dt>fracture: <a class="pgref" href="#Page_6">6</a></dt> -<dt>Franklin Mountains: <a class="pgref" href="#Page_18">18</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_G"><b>G</b></dt> -<dt>gadolinite: <a class="pgref" href="#Page_21">21</a>-22</dt> -<dt>garnet: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_36">36</a></dt> -<dt>gemstones, by kinds: <a class="pgref" href="#Page_18">18</a>-31</dt> -<dt>geodes, celestite: <a class="pgref" href="#Page_19">19</a></dt> -<dt>Georgia: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Gillespie County: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_30">30</a>, <a class="pgref" href="#Page_36">36</a></dt> -<dt>girdle facets: <a class="pgref" href="#Page_16">16</a></dt> -<dt>gneiss: <a class="pgref" href="#Page_22">22</a></dt> -<dt>Gonzales County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_29">29</a></dt> -<dt>grain: <a class="pgref" href="#Page_25">25</a></dt> -<dt>gram: <a class="pgref" href="#Page_7">7</a></dt> -<dt>graphite: <a class="pgref" href="#Page_30">30</a></dt> -<dt>Grimes County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>grinding: <a class="pgref" href="#Page_12">12</a></dt> -<dt>Guadalupe River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>Gulf Coast: <a class="pgref" href="#Page_25">25</a></dt> -<dt>Gulf Coastal Plain: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_35">35</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_H"><b>H</b></dt> -<dt>hardness: <a class="pgref" href="#Page_6">6</a></dt> -<dt>Hudspeth County: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_24">24</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_27">27</a>, <a class="pgref" href="#Page_28">28</a>, <a class="pgref" href="#Page_31">31</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_I"><b>I</b></dt> -<dt>index of refraction: <a class="pgref" href="#Page_5">5</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_J"><b>J</b></dt> -<dt>jasper: <a class="pgref" href="#Page_27">27</a>-28</dt> -<dt>Jeff Davis County: <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_27">27</a>, <a class="pgref" href="#Page_28">28</a>, <a class="pgref" href="#Page_31">31</a></dt> -<dt>jet: <a class="pgref" href="#Page_22">22</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_K"><b>K</b></dt> -<dt>Kinney County: <a class="pgref" href="#Page_28">28</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_L"><b>L</b></dt> -<dt>labradorite: <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_36">36</a></dt> -<dt>Lake Buchanan: <a class="pgref" href="#Page_21">21</a></dt> -<dt>Lampasas County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>lap plate: <a class="pgref" href="#Page_13">13</a></dt> -<dt>Lavaca County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Lee County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_29">29</a></dt> -<dt>lignite: <a class="pgref" href="#Page_22">22</a></dt> -<dt>Live Oak County: <a class="pgref" href="#Page_21">21</a></dt> -<dt>Llano County: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_19">19</a>, <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_30">30</a>, <a class="pgref" href="#Page_31">31</a>, <a class="pgref" href="#Page_40">40</a></dt> -<dt>Llano River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>luster: <a class="pgref" href="#Page_5">5</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_M"><b>M</b></dt> -<dt>Madeira topaz: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Mason County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_29">29</a>, <a class="pgref" href="#Page_30">30</a>, <a class="pgref" href="#Page_39">39</a>, <a class="pgref" href="#Page_40">40</a></dt> -<dt class="pb" id="Page_42">42</dt> -<dt>Massachusetts: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Maverick County: <a class="pgref" href="#Page_18">18</a></dt> -<dt>microcline: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_23">23</a>-24, <a class="pgref" href="#Page_29">29</a>, <a class="pgref" href="#Page_37">37</a></dt> -<dt>Mohs scale of hardness: <a class="pgref" href="#Page_6">6</a></dt> -<dt>moonstone: <a class="pgref" href="#Page_28">28</a></dt> -<dt>Mount Bonnell: <a class="pgref" href="#Page_19">19</a></dt> -<dt>muscovite: <a class="pgref" href="#Page_23">23</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_N"><b>N</b></dt> -<dt>natural glass: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_24">24</a>, <a class="pgref" href="#Page_28">28</a></dt> -<dt>Needle Peak, Presidio County: <a class="pgref" href="#Page_27">27</a></dt> -<dt>nivenite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>Nolan County: <a class="pgref" href="#Page_19">19</a></dt> -<dt>Nueces River: <a class="pgref" href="#Page_25">25</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_O"><b>O</b></dt> -<dt>obsidian: <a class="pgref" href="#Page_24">24</a>, <a class="pgref" href="#Page_29">29</a></dt> -<dt>Oklahoma: <a class="pgref" href="#Page_29">29</a></dt> -<dt>opal: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_24">24</a></dt> -<dt>opalized wood: <a class="pgref" href="#Page_35">35</a></dt> -<dt>orbicular jasper: <a class="pgref" href="#Page_28">28</a></dt> -<dt>ounce: <a class="pgref" href="#Page_7">7</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_P"><b>P</b></dt> -<dt>Packsaddle Mountain: <a class="pgref" href="#Page_23">23</a></dt> -<dt>Packsaddle schist: <a class="pgref" href="#Page_30">30</a></dt> -<dt>palm wood: <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_35">35</a></dt> -<dt>Palo Duro Canyon: <a class="pgref" href="#Page_21">21</a></dt> -<dt>pavilion: <a class="pgref" href="#Page_13">13</a>, <a class="pgref" href="#Page_16">16</a></dt> -<dd>facets: <a class="pgref" href="#Page_16">16</a></dd> -<dd>girdle facets: <a class="pgref" href="#Page_16">16</a>, <a class="pgref" href="#Page_17">17</a></dd> -<dt>pearl: <a class="pgref" href="#Page_24">24</a>-25</dt> -<dt>pegmatites and/or pegmatite dikes: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_29">29</a>, <a class="pgref" href="#Page_39">39</a></dt> -<dt>petrified wood: <a class="pgref" href="#Page_26">26</a></dt> -<dt>phantom crystals: <a class="pgref" href="#Page_26">26</a></dt> -<dt>pistacite: <a class="pgref" href="#Page_20">20</a></dt> -<dt>pleochroism: <a class="pgref" href="#Page_5">5</a></dt> -<dt>point: <a class="pgref" href="#Page_7">7</a></dt> -<dt>polishing: <a class="pgref" href="#Page_17">17</a></dt> -<dd>lap: <a class="pgref" href="#Page_13">13</a>, <a class="pgref" href="#Page_16">16</a></dd> -<dt>preformed stone: <a class="pgref" href="#Page_16">16</a></dt> -<dt>preforming: <a class="pgref" href="#Page_15">15</a></dt> -<dt>Presidio County: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_22">22</a>, <a class="pgref" href="#Page_24">24</a>, <a class="pgref" href="#Page_25">25</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_27">27</a>, <a class="pgref" href="#Page_28">28</a></dt> -<dt>properties of gemstones: <a class="pgref" href="#Page_5">5</a>-7</dt> -</dl> -<dl class="index"> -<dt class="center" id="index_Q"><b>Q</b></dt> -<dt>quartz: <a class="pgref" href="#Page_20">20</a>, <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_25">25</a>-28, <a class="pgref" href="#Page_29">29</a>, <a class="pgref" href="#Page_30">30</a></dt> -<dd>smoky: <a class="pgref" href="#Page_38">38</a></dd> -</dl> -<dl class="index"> -<dt class="center" id="index_R"><b>R</b></dt> -<dt>radioactive elements: <a class="pgref" href="#Page_22">22</a></dt> -<dt>radioactivity of gadolinite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>rarity: <a class="pgref" href="#Page_6">6</a></dt> -<dt>Reeves County: <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_27">27</a></dt> -<dt>Rio Grande: <a class="pgref" href="#Page_25">25</a></dt> -<dd>gravels of: <a class="pgref" href="#Page_21">21</a>, <a class="pgref" href="#Page_27">27</a>, <a class="pgref" href="#Page_38">38</a></dd> -<dd>Valley: <a class="pgref" href="#Page_27">27</a></dd> -<dt>rock crystal: <a class="pgref" href="#Page_26">26</a></dt> -<dt>rose quartz: <a class="pgref" href="#Page_26">26</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_S"><b>S</b></dt> -<dt>Sabine River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>sanding: <a class="pgref" href="#Page_12">12</a></dt> -<dt>sanidine: <a class="pgref" href="#Page_28">28</a></dt> -<dt>sawing: <a class="pgref" href="#Page_10">10</a></dt> -<dt>scheelite: <a class="pgref" href="#Page_20">20</a></dt> -<dt>schorl: <a class="pgref" href="#Page_30">30</a></dt> -<dt>size: <a class="pgref" href="#Page_7">7</a></dt> -<dt>“skill” facets: <a class="pgref" href="#Page_16">16</a></dt> -<dt>“slab” of gem materials: <a class="pgref" href="#Page_11">11</a></dt> -<dt>Smithsonian Institution: <a class="pgref" href="#Page_21">21</a></dt> -<dt>smoky quartz: <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_37">37</a></dt> -<dt>smoky topaz: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Spanish topaz: <a class="pgref" href="#Page_29">29</a></dt> -<dt>specific gravity: <a class="pgref" href="#Page_7">7</a></dt> -<dt>spinel: <a class="pgref" href="#Page_18">18</a>, <a class="pgref" href="#Page_28">28</a></dt> -<dt>star facets: <a class="pgref" href="#Page_17">17</a></dt> -<dt>step cut: <a class="pgref" href="#Page_15">15</a></dt> -<dt>streak: <a class="pgref" href="#Page_6">6</a></dt> -<dt>synthetic gems: <a class="pgref" href="#Page_7">7</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_T"><b>T</b></dt> -<dt>table facet: <a class="pgref" href="#Page_13">13</a>, <a class="pgref" href="#Page_15">15</a></dt> -<dt>tektite (bediasite): <a class="pgref" href="#Page_28">28</a>-29, <a class="pgref" href="#Page_39">39</a></dt> -<dt>tenacity: <a class="pgref" href="#Page_6">6</a></dt> -<dt>thorogummite: <a class="pgref" href="#Page_21">21</a></dt> -<dt>topaz: <a class="pgref" href="#Page_23">23</a>, <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_29">29</a>-30, <a class="pgref" href="#Page_39">39</a>, <a class="pgref" href="#Page_40">40</a></dt> -<dd>quartz: <a class="pgref" href="#Page_29">29</a></dd> -<dt>tourmaline: <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_29">29</a>, <a class="pgref" href="#Page_30">30</a>-31, <a class="pgref" href="#Page_40">40</a></dt> -<dt>Town Mountain, Llano County: <a class="pgref" href="#Page_26">26</a>, <a class="pgref" href="#Page_30">30</a></dt> -<dt>transparency: <a class="pgref" href="#Page_6">6</a></dt> -<dt>Travis County: <a class="pgref" href="#Page_19">19</a>, <a class="pgref" href="#Page_35">35</a></dt> -<dt>Trinity River: <a class="pgref" href="#Page_25">25</a></dt> -<dt>tumbled gems: <a class="pgref" href="#Page_17">17</a></dt> -<dt>turquoise: <a class="pgref" href="#Page_31">31</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_U"><b>U</b></dt> -<dt>U. S. National Museum: <a class="pgref" href="#Page_30">30</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_V"><b>V</b></dt> -<dt>value of gemstones: <a class="pgref" href="#Page_6">6</a>, <a class="pgref" href="#Page_7">7</a></dt> -<dt>Val Verde County: <a class="pgref" href="#Page_24">24</a></dt> -<dt>valverdites: <a class="pgref" href="#Page_24">24</a></dt> -<dt>Van Horn, Hudspeth County: <a class="pgref" href="#Page_27">27</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_W"><b>W</b></dt> -<dt>Walker County: <a class="pgref" href="#Page_29">29</a></dt> -<dt>Washington County: <a class="pgref" href="#Page_20">20</a></dt> -<dt>Webb County: <a class="pgref" href="#Page_21">21</a></dt> -<dt>weight, units of: <a class="pgref" href="#Page_7">7</a>, <a class="pgref" href="#Page_25">25</a></dt> -</dl> -<dl class="index"> -<dt class="center" id="index_Z"><b>Z</b></dt> -<dt>Zapata County: <a class="pgref" href="#Page_38">38</a></dt> -</dl> -<h2>Transcriber’s Notes</h2> -<ul> -<li>Silently corrected a few typos.</li> -<li>Renumbered figures 6 and 7 (and references to them) to correspond to their order in the printed book.</li> -<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li> -<li>In the text versions only, text in italics is delimited by _underscores_.</li> -</ul> - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of Texas Gemstones, by Elbert A. 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