Onyx is a banded variety of chalcedony, which is a type of quartz. Its bands are typically white and black.

Reddish brown and white onyx is known as sardonyx. Sardonyx was especially valued in Rome for seals, because it was said to never stick to the wax. The Roman General Publius Cornelius Scipio famously wore much fine jewellery made of sardonyx.

Onyx was particularly popular with the ancient Greeks and Romans. The name comes from the Greek word onux, meaning ‘fingernail’. Legend has it that one day Cupid, larking about with his arrow, accidentally cut the fingernails of Venus while she was sleeping on the sand. Cupid left the clippings scattered there and the fates turned them into stone so that no part of the divine body would ever perish. It is quite odd to think that Venus’s fingernails were black – in fact, they were probably creamy white, as in Greek times, almost all the many colours of chalcedony were called onyx and it was only later that the Romans narrowed the term to refer to black and dark brown only.

Black onyx is an especially effective backdrop for coloured stones, particularly opaque ones such as coral and jade, and as such is particularly popular in designer jewellery. Due to its fine texture, it is also ideal for carving, making it a popular material for today's lapidary artists. Onyx is also often used in fine jewellery to contrast with carved rock crystal and the deep red of rubies, particularly in Art deco designs.

The principal source of onyx is Brazil, but it is also available from parts of the USA, Germany, Mexico, India, and Africa.