The only proper use for the term onyx is to describe the opaque chalcedony composed of straight, parallels bands of black and white. The term is used incorrectly when applied to grayish chalcedony or agate, which has been dyed since ancient times to produce a material known as black onyx.
The name onyx comes from the Greek word onux, which means fingernail. The story is that one day Cupid cut the divine fingernails of Venus with an arrowhead 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 heavenly body would ever perish.
Black isn't normally the color one associates with fingernails, but for the Greek almost all colors of chalcedony, from fingernail white to dark brown and black, were called onyx.
Later, the Romans narrowed the term to refer to the black and dark brown colors of chalcedony only.
Onyx with reddish brown and white bands is known as sardonyx. Sardonyx was highly valued in Rome, especially for seals, because it was said to never stick to the wax. The Roman General Publius Cornelius Scipio was known for wearing lots of sardonyx.
Sardonyx alternates with peridot as the birthstone for the month of August.