Onyx is a black and sometimes white banded agate. It should not be confused with onyx marble, which is limestone. Onyx takes its name from the Greek word "onux", which means claw of the hand or fingernail. Onyx deposits are found in countries all over the world. The stoneĀ is a fairly tough substance with a hardness of 6-7 on the Mohs scale; but still requires proper care to keep it in good condition.