It is found in nature in several forms, including quartz and opal. In fact, silica has 17 crystalline forms. Also, horsetail is one of the plants which is rich in Silica.
It is manufactured in several forms including glass (in colorless high purity form called fused silica) and silica gel (used e.g. as desiccants in brand new clothes and leather goods). Silica is a major ingredient of Portland cement. The ceramic re-entry heat protection tiles mounted on the bottom side of the Space Shuttles are made mostly of silica.
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.
Silica is also used as a food additive, primarily as a flow agent in powdered foods, or to absorb water.