The Onyx reference article from the English Wikipedia on 24-Jul-2004
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Onyx

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This article is about the mineral. See Onyx (disambiguation) for other meanings

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
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