Silicon dioxide, also known as silica, is a common compound that occurs naturally in several crystalline forms such as quartz, cristobalite, and tridymite. It has the molecular formula of SiO2 and is a three dimensional covalent solid where each silicon atom is tetrahedrally bonded to four oxygen atoms. Silica is relatively inert due to its very strong Si-O bonds and has various industrial and commercial uses such as a drying agent, support for chromatographic materials, in toothpaste as an abrasive, and as a refractory material in high-temperature fabrics.