The chemical compound silica, also known as silicon dioxide, is the oxide of silicon, chemical formula SiO2. Siliceous is an adjective meaning “referring to silica”.
Silica is found in nature in several forms, including quartz and opal. In fact, it has 17 crystalline forms. The most common constituent of sand in inland continental settings and non-tropical coastal settings is silica, usually in the form of quartz because the considerable hardness of this mineral resists erosion. However, the composition of sand varies according to local rock sources and conditions.