Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony. The colors of its bands range from black to almost every color. 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 morganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.