Part 1

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.

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.

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.