Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand.  Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as a synthetic product.  Notable examples include fused quartz, fumed silica, silica gel, opal and aerogels.  It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.It represents more than 60% of the mass of the earth's crust.  Silica SiO2 is chemically stable and hardly attacked by hydrofluoric acid, a reaction used in glass etching. Each of the crystallized forms has several varieties.  The transition from one species to another results in dimensional variations, some of which are important at the scale of ceramic materials.   This often results in thermal shock fragility and other phenomena due to expansion tension.