fused silica
Fused silica is glass stripped to its purest essence: almost 100 percent silicon dioxide, SiO2, and nothing else. It is the network former with no modifiers, no flux, no additives, so its structure is a single, fully connected random network in which every oxygen bridges two silicons and there are no broken bonds anywhere. That total connectivity is the source of everything remarkable and everything inconvenient about it. It is the benchmark against which all other oxide glasses are measured.
Because the network is unbroken, fused silica has the extreme properties you would expect at the far end of the family. Its thermal-expansion coefficient is astonishingly low, around 0.5 x 10^-6 per degree C, more than ten times lower than soda-lime glass, so it barely notices temperature changes and shrugs off ferocious thermal shock: you can heat it white-hot and plunge it into water without cracking. Its softening and working temperatures are correspondingly enormous, well above 1600 degrees C, because there are no modifier-cut bonds to soften. It is also exceptionally transparent, deep into the ultraviolet, and chemically about as durable as an oxide glass can be.
The price of that perfection is that fused silica is difficult and expensive to make and to shape, precisely because it stays so stiff to such high temperatures. So it is reserved for jobs where nothing cheaper will do: the core and cladding of optical fibres that carry the internet, crucibles for growing silicon single crystals, ultraviolet lamp envelopes and lenses, and ultra-stable mirror blanks. A small note on names: fused silica usually means glass melted from a pure synthetic or chemical source, while fused quartz means glass melted from ground natural quartz, which carries slightly more impurities; both are the same amorphous SiO2 at heart.
Every strand of optical fibre threading the planet is essentially fused silica, drawn into a hair-thin thread so pure and so low-loss that light can travel tens of kilometres before it needs a boost. No modifier-laden glass could carry a signal that far; it is the flawless, fully bridged silica network that keeps the light clean. The same material's near-zero expansion is why it also makes the mirror blanks for gravitational-wave detectors.
Its unbroken, ultra-pure network gives fused silica the lowest expansion, highest working temperature, and cleanest optics of any common glass.
Fused silica is amorphous SiO2, not crystalline quartz, even when it is made by melting quartz. Do not confuse fused silica or fused quartz (a glass) with crystalline quartz (a mineral); they share a formula but not a structure.