Gemstone

The term gemstone refers to a valuable, and often beautiful rock, usually, but not always, composed of a single crystal of a mineral.

Gemstones have industrial uses, other than jewelry. Analog watches exploited the hardness of gemstones to serve as long-lasting bearings. Diamonds are the hardest known crystals, and diamond dust is used as an abrasive for cutting other hard substances. Saws for cutting rocks used blades coated with diamond dust.

Some gemstones have other very useful optical qualities. Viking explorers used gemstone crystals for navigating on foggy days that allowed them to detect the location of the sun through polarization.

Piezoelectricity is a property of some gemstone crystals, which generate small amounts of electicity when squeezed. Microphones and audio speakers use this property.

Some early lasers relied on optically pure crystals of the gemstone ruby, to generate rays of laser light.

Jet is an example of a substance, widely called a gem, which is not crystaline. Jewellers polish coal, which is almost entirely pure carbon, but is not a crystal, to mount in jewelry.