The group of minerals represented by the chemical composition SiO 2 occur in many forms. The most common form is crystalline quartz which is essential constituent of many igneous, sedimentary and metamorphic rocks. Cryptocrystalline form of silica is called chalcedony which is usually the mineral which makes up the sedimentary rock chert .
Amorphous opal is a silica mineral deposited by near-surface groundwater in arid regions.
Quartz Quartz is a tectosilicate with each SiO 4 tetrahedron sharing all four oxygens with adjacent tetrahedra. Besides, we have high pressure equivalent minerals - coesite, stishovite and keatite. We will discuss only the low temperature form quartz . Crystallography Trigonal Chemistry SiO 2 Physical Properties Color of quartz varies highly. The common quartz crystals are colorless or white.