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. Hardness: 7, Specific gravity: 2.65-2.66, Optical character: Uniaxial, Chemical composition and name: Silicon dioxide, SiO_2.  

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