Onyx is the most common and least expensive black gemstone. It is a banded variety of chalcedony, a form of silica made up of layers of quartz and moganite. Onyx stones often have bands of white, red or brown.
Most onyx used in jewelry is dyed to produce a consistent black color and is shaped and polished as a cabochon or cut into beads. The stone may be engraved or carved as a cameo. Sardonyx is a variation of onyx in which the colored bands are shades of red. The gemstone is found in porous lava rock in Brazil, Uruguay, Pakistan, Sri Lanka, India, Hawaii and Madagascar.