Rock Crystal
From The Gemology Project
Revision as of 13:49, 20 January 2007 by Tom Goodwin, G.G. (talk | contribs)
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. The genuine "crystal balls," used 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 large diameters. The Ancient Greeks prized rock crystal and believed it to be water frozen by the gods to a state of perpetual solidity.