Silicon dioxide

Silicon dioxide, also known as silica, is a chemical compound that is an oxide of silicon with the chemical formula SiO2. It has been known for its hardness since ancient times. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms. Silica is manufactured in several forms including fused quartz, crystal, fumed silica (or pyrogenic silica), colloidal silica, silica gel, and aerogel.

Athens

Geothermal gradient