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.