Ancient thinkers believed quartz was ice frozen so hard it could never melt
The Greek word for crystal comes from their word for icy cold, because philosophers such as Theophrastus thought clear quartz was supercooled ice. Pliny the Elder agreed, noting it turned up near Alpine glaciers but not in hot places. The idea survived until at least the 17th century, when quartz helped launch modern crystallography instead.
Quartz is silicon dioxide, built from a continuous framework of silicon-oxygen tetrahedra in which every oxygen atom is shared between two of them. That makes it a framework silicate by structure and an oxide by composition. It is the second most abundant mineral in Earth's lithosphere, around 12 percent by mass, and it sets the value of 7 on the Mohs hardness scale.
The turning point in understanding came from Nicolas Steno in the 17th century. He noticed that however large or oddly shaped a quartz crystal was, its long prism faces always met at exactly 60 degrees. That observation became the law of constancy of interfacial angles and paved the way for crystallography. The ideal crystal is a six-sided prism capped at each end by pyramid-like points, though real specimens are often twinned, distorted, intergrown or shapeless.
Quartz has two forms. Ordinary alpha-quartz is trigonal; above 573 degrees Celsius at normal pressure it abruptly becomes the more symmetrical, hexagonal beta-quartz. The shift involves only a slight rotation of the tetrahedra, but it changes the volume enough to crack ceramics during firing, damage ornamental stone after a fire and fracture crustal rocks. Beta-quartz cannot survive at room temperature, so any quartz you can hold is alpha. Both forms are chiral, existing in left- and right-handed versions even though their building blocks are not.
The name has two proposed origins. One traces it through German Quarz to a Polish dialect word related to the Czech for hard; another links it to a Saxon term meaning cross-vein ore. Whatever its roots, quartz has been the most widely used mineral in jewellery and hardstone carving since antiquity, especially in Europe and Asia, and several varieties count as gemstones.
Source: Quartz
