quartz one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO 2 . It occurs in crystals of the hexagonal system, commonly having the form of a six-sided prism terminating in a six-sided pyramid; the crystals are often distorted and twins are common. Quartz may be transparent, translucent, or opaque; it may be colorless or colored. Varieties are classified as crystalline and cryptocrystalline. Crystalline varieties include ordinary colorless crystallized quartz, or rock crystal; rose quartz; yellow quartz, sometimes used as imitation topaz ; smoky quartz, or cairngorm stone; milk-white milky quartz; aventurine quartz, which contains scales of hematite or mica; and amethyst . Varieties of cryptocrystalline quartz, the crystal structure of which can be seen only under the microscope, if at all, are chalcedony , flint , hornstone, and chert. Colored varieties of chalcedony known by special names are carnelian , sard, chrysoprase, agate , onyx , sardonyx, and jasper . Clastic quartzes are sand and sandstone .
