Fused quartz or fused silica is glass consisting of silica in a non-crystalline form. It differs from traditional glasses in containing no other ingredients, which are typically added to glass to lower the melt temperature. Although the terms quartz and fused silica are used interchangeably, the fundamental structures and creation of each glass differ. Fused silica has extremely high working and melting temperatures. The optical and thermal properties of fused quartz are superior to those of other types of glass due to its purity. These traits make fused quartz and fused silica a perfect candidate for the semiconductor and solar industries. It transmits ultraviolet better than other glasses, so is used to make lenses and optics for improved transparency in the ultraviolet spectrum. Its low coefficient of thermal expansion also makes it a useful material for precision mirror substrates.

The extremely low linear coefficient of thermal expansion, about 5.5×10−7/°C (20–320°C), accounts for its remarkable ability to undergo large, rapid temperature changes that can withstand some of the most rigorous processes.