Rock Crystal
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Rock Crystal        

-> variety of quartz, silicon dioxide.
-> colorless.
-> index of refraction: 1.544 - 1.553.
-> birefringence: 0,009.
-> specific gravity (density): 2.65 - 2.66.
-> hardness: 7.
-> cleavage: none.
-> optic sign: positive.
-> gloss: glass gloss.
-> break: shell.
-> crystal system: trigonal.

-> Widely used as a gemstone; one of lest expensive gemstones.
-> Carvings: great fortune teller crystal ball.
-> Sometimes used to imitate a diamond, though it lacks its fire.
-> Comes from "krystallos" (Greek: ice); it was believed to be permanently freezed water.
-> Single crystals may be even four metres long!
-> Major suppliers: USA, Brazil, Switzerland.