Crystallised silica—silicon dioxide—constituting roughly 12 per cent of the Earth's crust. One of the most common minerals on the planet, yet fully transparent crystals are limited in supply. A ubiquitous composition, but the conditions for true clarity are demanding.

The ancient Greeks named it "krystallos"—ice—believing it to be frozen water that would never melt. That misunderstanding became the origin of the word "crystal."