Rock Crystal

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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. 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.