Onyx

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This article is about the mineral. For other uses, see Onyx (disambiguation).

Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. The colors of its bands are white and black. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black.

Image:Agate banded 750pix.jpg It is usually cut as a cabochon, or into beads, and is also used for intaglios and cameos, where the bands make the image contrast with the ground. 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 - 2.667
Refractive index (R.I.)	1.543 - 1.552 to 1.545 - 1.554
Birefringence	0.009
Optic sign	Positive
Optical character	Uniaxial

[edit] Culture and historical/mythical usage

According to Rebbenu Bachya, the word Shoham in the verse Exodus 28:20 means "Onyx" and was the stone on the Ephod representing the Tribe of Joseph. There were also two Shoham stones on the shoulders of the Ephod.

In the Dreamlands, a fictional location in the works of the author H. P. Lovecraft, the palaces of Kadath are built of onyx.