silicon

A brittle metalloid element that exists in two allotropic forms; occurs principally in sand, quartz, granite, feldspar, and clay. It is usually a grey crystalline solid but is also found as a brown amorphous powder. It is used in transistors, rectifiers, solar cells, and alloys. Its compounds are widely used in glass manufacture, the building industry, and in the form of silicones. (Source: CED)