Onyx is a banded variety of chalcedony. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white. Onyx is formed of bands of chalcedony in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz  and moganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates. Sardonyx is a variant in which the colored bands are  sard (shades of red) rather than black.