crystalline silica, SiO2, occurring either in prisms capped by rhomnohedra (low-temperature quartz, stable up to 573°C) or in heaxagonal bipyramidal crystals (high-temperature quartz, stable above 573°C). Widely distributed in rocks of all kinds; igneous, metamorphic and sedimentary; usually colourless and transparent (rock crystal), but often coloured by minute quantities of impurities as in citrine, cairngorm etc; also finely crystalline in the several forms of chalcedony, jasper etc.
