Rock crystal is the name given to all clear colorless forms of the mineral quartz. As such it is related to amethyst which is the purple form of quartz, and to citrine which is the yellow variety.  It owes its name to the ancients Greeks who found it in caves near Mount Olympus, and called it krustallos meaning ice, as they believed it to be water which had been permanently frozen by the gods. Quartz's high thermo-conductivity, which makes it feel cool to the touch, may have added to this belief.

Rock crystal is, like all quartz, formed from the two most abundant elements in the earth's crust: silicon and oxygen. It is brittle, so it'll chip and crack easily. Just as other quartz varieties, it occurs in quartz veins where it crystallizes inside rock cavities. It is also common in pockets in pegmatite dikes. Rock crystal often has minute cavity inclusions containing carbon dioxide or water. Rock crystal is recognized by its crystal habit, transparency, hardness, glassy luster, conchoidal fracture and general lack of cleavage.

Rock crystal is in wide use as a gemstone due to its beauty, affordability, availability and ease of cutting. Historical records show the use of rock crystal for decoration and jewelry for at least 4,000 years. Tools and weapons were made from rock crystal long before it was used for decorative purposes, and people make utensils of rock crystals like vases until today. Cut rock crystal has been used as imitation for diamond. It is also widely used as a popular ornamental stone. The most well known ornamental carving for rock crystal is the crystal ball of fortune telling fame. There are many fine chandeliers that are outfitted with rock crystal ornaments.

The best known rock crystal sources are in India and Sri Lanka, there are sites in Kenya, Madagascar and Brazil. Large individual crystals of quartz have been found in Brazil, the largest weighs over 44 tons. Many occurrences of rock crystal are found in USA, especially in the Hot Springs area of Arkansas, as well as in Russia (Ural Mountains and Sakha Republic). In Europe, the mineral has been found in Switzerland.

Characteristics:

Chemistry: SiO2, Silicon Dioxide
Hardness (Mohs scale): 7
Density:  2.65
Refractive index: 1.544 - 1.553
Dispersion: 0.013
Crystal system: trigonal
Cleavage: none
Fracture: conchoidal