Onyx is formed of bands of chalcedony in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Onyx is a very rare material, with very complex natural designs. Because of its limited availability Onyx tends to be very expensive with prices starting at $35 per sqft for the raw material. 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. Black onyx is perhaps the most famous variety, but is not as common as onyx with colored bands. Artificial treatments have been used since ancient times to produce both the black color in black onyx and the reds and yellows in sardonyx. Most black onyx on the market is artificially colored.