The most common form of silicon dioxide is crystalline quartz, which is hard, brittle and transparent, with a high melting temperature. It is only one form of silicon dioxide, however, and the compound's properties depend somewhat on the form in which it is found.
Silicon dioxide, or silica, is the major constituent of several types of crystals, and it is also the major constituent in commercial glass. Pure silica glass, known as fused silica, can also be created, but its high-melting temperature restricts it to specialized applications. Fused silica has very little thermal expansion and good thermal shock resistance, and it is very chemically inert. While quartz crystals are extremely regular in their molecular structure, with each molecule oriented in a predictable fashion to those around it, fused silica does not have a consistent orientation of silica molecules to one another.
Silicon dioxide is a compound of the two most common elements in the Earth's crust, silicon and oxygen. Various forms of silicon dioxide make up 59 percent of the Earth's crust and 95 percent of all known rocks. Its commonality and chemical inertness have made it very important in human industry. Optical fibers for telecommunication are primarily made from silica, and it is the primary raw material for several ceramics.