Diamonds crystallize in the isometric system (see crystal ) commonly as transparent to translucent white, colorless, yellow, green, blue, pink, or brown octahedrons (the familiar diamond shape). The extraordinary brilliancy of diamonds after faceting is due to their very high refractive index, which is greater than that of any other naturally occurring gemstone. In addition to the gem varieties there are bort, which is poorly crystallized or of inferior color and in fragmentary condition, and carbonado (black diamond), which is gray to black and opaque, with poor cleavage. Bort and carbonado are used as abrasives, in the cutting of diamonds, and for the cutting heads of rock drills. Diamond abrasives may have been used as early as 2500 BC in China.
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