|Onyx usually refers to a black agate that may or may not have banding. The name comes from the Greek word "onux", which means fingernail. It is tied to an old Greek myth.
Since there is a high demand for good quality material, some black "onyx" on the market is now agate dyed black. It is not easy to ascertain whether it was originally black or dyed. Some material sold as onyx is actually a carbonate and much softer than true onyx.
The "Mexican Onyx" used in carved chess sets from Mexico is actually a form of marble and not true onyx at all.