The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2.
It is found in nature in several forms, including quartz and opal. In fact, silica has 17 crystalline forms. Also, many forms of life include silica structures, including microorganisms such as diatoms, plants such as horsetail, and animals such as hexactinellid sponges.
It is manufactured in several forms including glass (in colorless high purity form called fused silica), synthetic amorphous 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, as are the firebricks used in steel processing.
Inhaling crystalline silica dust can lead to silicosis.