Black onyx is a form of chalcedony, which is a variety of quartz. Black onyx or black agate is most commonly recognized as a solid black stone although it will often be mixed with white lines or banding running through it. This banding can create some unusual patterns within the stone. Whether solid black or striped, the stone is prized for its pure black sheen and high polish. The stone is most commonly used for jewellery.

Black onyx was used for jewellery by many ancient cultures. It has been found in tombs in ancient Egypt, and onyx is mentioned in the Bible.