\Quartz\, n. [G. quarz.] (Min.)
A form of silica, or silicon dioxide ({SiO2}), occurring in
hexagonal crystals, which are commonly colorless and
transparent, but sometimes also yellow, brown, purple, green,
and of other colors; also in cryptocrystalline massive forms
varying in color and degree of transparency, being sometimes
opaque.
Note: The crystalline varieties include: amethyst, violet;
      citrine and false topaz, pale yellow; rock crystal,
      transparent and colorless or nearly so; rose quartz,
      rosecolored; smoky quartz, smoky brown. The chief
      crypto-crystalline varieties are: agate, a chalcedony
      in layers or clouded with different colors, including
      the onyx and sardonyx; carnelian and sard, red or
      flesh-colored chalcedony; chalcedony, nearly white, and
      waxy in luster; chrysoprase, an apple-green chalcedony;
      flint, hornstone, basanite, or touchstone, brown to
      black in color and compact in texture; heliotrope,
      green dotted with red; jasper, opaque, red yellow, or
      brown, colored by iron or ferruginous clay; prase,
      translucent and dull leek-green. Quartz is an essential
      constituent of granite, and abounds in rocks of all
      ages. It forms the rocks quartzite (quartz rock) and
      sandstone, and makes most of the sand of the seashore.