Texas Gemstones

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.

Part 2

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.

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