Onyx
Onyx, sard and sardonyx are all varieties of chalcedony. Onyx is similar to agate but it has straight rather than curved bands. These may be brown and white or black and white. Sard is a brownish red variety similar to agate. Sardonyx, a blend of sard and onyx, has the straight wide bands of onyx and the brownish red of sard.
Onyx was very popular with the Ancient Greeks and Romans. The name comes from the Greek word 'onyx' which means nail or claw. Ancient myth tells of how Eros (Cupid) cut the fingernails of Aphrodite (Venus) while she was sleeping. He left the clippings scattered on the sand and the fates turned them into stone so that no part of the divine body would ever deteriorate. Black is not a colour usually associated with healthy fingernails, but in Greek times almost all the colours of chalcedony from white to dark brown and black were called onyx. It was the Romans who narrowed the term to refer only to black and dark brown.
Onyx which is a reddish brown and white is known as sardonyx. Sardonyx was highly valued in Rome, especially for seals, as it was said never to stick to the wax. Onyx, Sard and Sardonyx are carved as small sculptures and intaglios or may be cut and polished as beads. Onyx was often used in Art Deco designs as a striking contrast with carved rock crystal and rubies. It is also popular in marcasite jewellery.
Since Ancient Egyptian times onyx has been stained to improve or change its colour. Onyx has often been produced by soaking agate in a sugar solution, then heating it in sulphuric acid to carbonize the sugar. Virtually all onyx sold today is chalcedony dyed opaque black rather than natural onyx. Sard may be imitated by saturating the chalcedony with an iron solution.
They are found worldwide formed by deposits of silica in gas cavities in lavas, which results in the distinctive bands.