Commercial glasses may be divided into soda–lime–silica glasses and special glasses, most of the tonnage produced being of the former class. Such glasses are made from three main materials—sand (silicon dioxide, or SiO₂), limestone (calcium carbonate, or CaCO₃), and sodium carbonate (Na₂CO₃). Fused silica itself is an excellent glass, but, as the melting point of sand (crystalline silica) is above 1,700 °C (3,092 °F) and as it is very expensive to attain such high temperatures, its uses are restricted to those in which its superior properties—chemical inertness and the ability to withstand sudden changes of temperature—are so important that the cost is justified. Nevertheless, the production of fused silica glass is quite a large industry; it is manufactured in various qualities, and, when intended for optical purposes, the raw material used is rock crystal rather than quartz sand.