Silicon

Silicon exists in the Sun, stars, and in meteorites. It is found in plants and in animal bones. In the Earth's crust, there are at least 500 minerals—substances with definite chemical compositions and crystal forms. More than a third of these compounds contain silicon and oxygen.

Silicon and oxygen form silicon dioxide, SiO₂, (as known as silica). Sand is mostly silica with some contributions from shells and corals. When mixed with lime (calcium oxide, CaO), soda (sodium carbonate, Na₂CO₃) and trace substances, then melted in a furnace, silica become the key component of glass.

The purest form of silica, SiO₂, is quartz, a common mineral that is found as nearly colorless crystals. Slightly impure quartz makes crystals of amethyst (purple or violet), opal (translucent, milky) and agate (striped), all of which are prized for their aesthetic value.

Practically all the rocks and clays contain silicon and oxygen combined chemically with metallic elements in compounds called silicates. A common exception is limestone, which is calcium carbonate.