Topaz. A silicate of aluminum and fluorine. Orthorhombic crystals common, usually prisms capped at one end by pyramided faces and abruptly terminated at the other. Colorless when pure, but often variously colored due to impurities. Very exceptionally hard (8 in the scale); specific gravity, 3.5. One good cleavage across the prism zone; usually found as crystals in, and in cavities in, igneous rocks. Appears always to have formed from highly heated vapors or liquids given off by cooling molten rock masses. Topaz is one of the more highly prized of the gem stones. Tourmaline. Composition, very complex, but chiefly a silicate of boron and several metals and semimetals. Commonly as crystals in the trigonal system in both long and short prismatic forms, as shown by Figure 75m, with opposite ends not unlike. Extra hard (7 in the scale); specific gravity, about 3. Color, widely various, but brown and black are most common. Practically no cleavage. Tourmaline probably always originated as a high temperature mineral, especially as crystals in granites and related rocks and in certain metamorphic rocks which have been subjected to high temperature and pressure. Certain transparent colored varieties of tourmaline rank high among the semiprecious stones. Turquoise. A hydrous phosphate of aluminum. Massive noncrystalline, blue to green, waxy luster, mostly opaque, hardness of 6, and specific gravity of about 2.7. Turquoise is a high temperature mineral found in veins and cavities in certain igneous rocks. It is a rare mineral used as a gem stone. Zircon. A silicate of zirconium usually crystallized in the tetragonal system as simple four-sided prisms capped by four-sided pyramids. (See Figure 75l.) Very poor cleavages. Color usually brown. Hardness, 7.5 (extra high); specific gravity, nearly 4.7. Brilliant luster. Zircon is very commonly present as scattering crystals of varying size in most igneous rocks. Also common as crystals in various metamorphosed stratified rocks, and less common in some sand and gravel deposits. Certain transparent varieties, especially the brown and pink ones called _hyacinth_, are used as gem stones. Zircon is also the source of oxide of zirconium used in making mantles for certain incandescent lights. CHAPTER XXI ECONOMIC GEOLOGY