Quartz, silicon dioxide, is one of the most common minerals. It is glassy, waxy, greasy, or dull and is transparent or translucent. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has a specific gravity of 2.65. The curved, conchoidal fracture shown by many specimens helps identify it. Quartz is plentiful in Texas. It occurs in igneous rocks, such as granite, llanite, and pegmatite; in metamorphic rocks, such as quartzite, schist, and gneiss; and in sedimentary rocks, such as some sandstone, conglomerate, and breccia. [Illustration: Quartz crystal, with inclusions, from Burnet County, Texas.] Quartz is found as crystals and as masses. Some of the masses are coarsely crystalline, but some are made up of extremely small crystalline particles called _cryptocrystalline_ quartz. Some of the cryptocrystalline varieties of quartz found in Texas are chalcedony, chert, and jasper. Some of the coarsely crystalline varieties found here are amethyst, milky quartz, rose quartz, smoky quartz, and rock crystal. [Illustration: Amethyst geode from the Alpine area of Brewster County, Texas.] A colorless, glassy variety of quartz, called _rock crystal_, is clear enough to see through. It is found as crystals that are 6-sided prisms with pyramid-like faces on the ends. This variety is commonly associated with igneous rocks, such as those of the Llano uplift area of central Texas and of the Trans-Pecos country of west Texas. It is commonly used as a gemstone and is made into necklaces, earrings, and other jewelry. Some specimens of rock crystal have slender, needle-like crystals of other minerals, such as tourmaline, actinolite, or rutile, enclosed in them. A clear, glassy variety of quartz, _amethyst_, has a purple or violet color. It, like rock crystal, is commonly found in 6-sided prisms with pyramid-shaped ends and is also prized as a gemstone. Amethyst has been found in Precambrian rocks in the Llano uplift area of central Texas. (Amethyst Hill, a locality well known to collectors for many years, is in northeastern Gillespie County.) In west Texas, amethyst has been found in Cenozoic igneous rocks in the Sierra Blanca and Quitman Mountains of Hudspeth County and in the Alpine area of Brewster County. [Illustration: Milky quartz from Burnet County, Texas.]