Silicon dioxide (SiO2) which occurs naturally in three main forms: (a) crystalline silica includes the minerals quartz, tridymite, and cristobalite; (b) cryptocrystalline or very finely crystalline silica includes some chalcedony, chert, jasper, and flint; and (c) amorphous hydrated silica includes opal, diatomite, and some chalcedony. Coesite and stishovite are two high-density polymorphs of quartz which rarely occur in nature but have been synthesized experimentally.