Main Page | See live article | Alphabetical index 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 For information about Specific Gravity, see Density ''See also: List of minerals. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.