any of various crystallized forms of silica that are transparent, translucent, or colored, used in many ways, esp. as a gem or in making glass, lenses, or quartz crystals
A very hard mineral composed of silica, SiO2, found worldwide in many different types of rocks, including sandstone and granite. Varieties of quartz include agate, chalcedony, chert, flint, opal, and rock crystal.