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Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. The colours of its bands are white and black. It is usually cut as a cabochon, or into beads, and is also used for intaglios and cameos. Some onyx is natural but much is produced by the staining of agate.

Technical Details
Chemical Composition and Name SiO2 - Silicon dioxide
Hardness (Mohs Scale) 7
Specific Gravity* 2.65 - 2.66
Refractive index (R.I.) 1.543 - 1.552 to 1.545 - 1.554
Birefringence 0.009
Optic Sign Positive
Optical Character Uniaxial

''See also: List of minerals.