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.