Rock crystal is the colorless and transparent variety of the quartz family. It is also the most widely distributed type of quartz and mineral of the Earth’s crust. The crystals are usually found as colorless hexagonal prisms, with pyramidal ends and striations perpendicular to their length. These are often twinned. The cleavage of the rock crystal is poor and fracture is conchoidal.
The name ‘quartz’ comes from the Greek word krustallos, meaning ice. It was thought that quartz was ice formed by the gods. Since the Middle Ages, crystal balls made of rock crystal have been used to predict the future. Today, rock crystal is used in lamps, lenses, and the manufacture of glass and prism instruments. Synthetic rock crystal has been produced since 1950 for use in watches.
Occurrence
Rock crystal is found worldwide but the most important source of the stone is Brazil. Other localities include Pakistan, Madagascar, the USA, the former USSR, and the French and Swiss Alps.
Rock Crystal Properties
- Specific Gravity: 2.65
- Refractive Index: 1.54-1.55
- Birefringence (DR): 0.009
- Luster: Vitreous
- Crystal Structure: Trigonal
- Composition: Silicon dioxide
- Hardness: 7½
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