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.