Onyx is a black variety of massive quartz (i.e. chalcedony), which may or may not also possess parallel white banding. However, most of the “black onyx” sold within the marketplace today is white or light gray chalcedony that has been “carbonized” in a sugar solution to turn it black. In addition, many other so-called “onyx” stones are not even chalcedony at all, with banded limestone being the most commonly encountered simulant.

The ancient Greeks and Romans prized onyx, as the black and white regions provided two contrasting colors for the carving of cameos and intaglios. The material was also very suitable for the art deco design style that was popular during the twentieth century’s interwar period, but the above-mentioned abuse of the name has since somewhat diminished the material’s reputation and value.