Rock Crystal

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Revision as of 14:54, 20 January 2007 by Tom Goodwin, G.G. (talk | contribs) (Added "sometimes" as a description of meager frequency of use as compared to glass imitations.)
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Rock Crystal (Colorless Quartz)
Chemical composition SiO2 Silicon dioxide
Crystal system Trigonal
Habit Prismatic
Cleavage Poor
Fracture Conchoidal
Hardness 7
Optic nature Uniaxial +
Refractive index 1.544 - 1.553
Birefringence 0.009
Dispersion Low, 0.013
Specific gravity 2.65
Lustre Vitreous
Pleochroism None

Rock Crystal is the common name for colorless quartz. The material is abundant and has been used for centuries as a gem.There are large variations in size, ranging from small polished beads to massive carvings. The genuine "crystal balls," used sometimes in "fortune-telling" are composed of rock crystal. These quartz spheres are known to have been ground and polished in Japan. They rarely have a diameter exceeding 7.62 centimeters. These spheres are usually imitated by polished glass of larger diameters. The largest known crystal ball is displayed at the Smithsonian Institution in Washington, D.C. It is called the "Warner Crystal Ball." This specimen has a weight of 48.5 kilograms and a diameter of 32.5 centimeters. The Ancient Greeks prized rock crystal and believed it to be water frozen by the gods to a state of perpetual solidity. They carved the material into seals and intaglios.