Silicon dioxide

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. 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 Shuttless are made mostly of silica.

Inhaling silica dust can lead to silicosis.

For the definitive read on the chemistry of silica, see RK Iler "The Chemistry of Silica" ISBN 047102404X, it's a fascinating read, but expensive.