one of the commonest minerals, silicon dioxide, SiO 2 , having many varieties that differ in color, luster, etc., and occurring either in masses (as agate, bloodstone, chalcedony, jasper, etc.) or in crystals (as rock crystal, amethyst, citrine, etc.)

the chief constituent of sand and sandstone, and an important constituent of many other rocks. It is piezoelectric and used to control the frequencies of radio transmitters.