Onyx is a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color (save some shades, such as purple or blue.) Commonly, specimens of onyx available contain bands of colors of white, tan, and brown. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black. Pure black Onyx is common, and perhaps the most famous variety, but not as common as Onyx with banded colors.
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.