Elbert A. King

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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.
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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 25 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. ”

“ 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) . ”

“ 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. ”

“ 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 281⁄3 grams is equal to 1 ounce avoirdupois. ”