Black onyx is a form of chalcedony, which is a variety of quartz. Black onyx is most commonly recognized as a solid black stone although it will often be mixed with white lines or banding running through it.
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. The stone is believed to relieve stress and enhance self-control. It is also an alternate birthstone for February.