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. Dake, H. C., Fleener, F. L., and Wilson, B. H. (1938) Quartz family minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York. Ford, W. E. (1932) A textbook of mineralogy (4th ed.): John Wiley and Sons, Inc., New York. Kraus, E. H., and Slawson, C. B. (1947) Gems and gem materials (5th ed.): McGraw-Hill Book Company, Inc., New York. Kunz, G. F. (1892) Gems and precious stones of North America (2d ed.): Scientific Publishing Company, New York. Pough, F. H. (1953) A field guide to rocks and minerals: Houghton Mifflin Company, Boston. Simpson, B. W. (1958) Gem trails of Texas: Granbury, Texas. Sinkankas, John (1955) Gem cutting: D. Van Nostrand Company, Inc., Princeton, New Jersey. —— (1959) Gemstones of North America: D. Van Nostrand Company, Inc., Princeton, New Jersey. Smith, G. F. H. (1958) Gemstones (13th ed.), revised by F. C. Phillips: Methuen and Company, Ltd., London. Sperisen, F. J. (1950) The art of the lapidary: The Bruce Publishing Company, Milwaukee, Wisconsin.