a glassy, very hard mineral, silicon dioxide, SiO, found in a variety of forms, as in quartz, opal, chalcedony, sand, or chert
See silica in American Heritage Dictionary 4
(sĭlˈĭ-kə)
noun
A white or colorless crystalline compound, SiO2, occurring abundantly as quartz, sand, flint, agate, and many other minerals and used to manufacture a wide variety of materials, especially glass and concrete.