Onyx is part of the agate family and has long been used in jewellery. with its coal black colour and ability to take a high polish it was often used as a substitute for sapphire in much art deco jewellery. It was also used as an imitator of jet. Not to be confused with yellowish/green stone often used for ornaments, cigarette lighters and table tops which is  called Algerian onyx, but is in fact calcite.