How can I tell if black onyx is real?

One way to tell if your black onyx is real or not is to use a scratch test. Because black onyx clocks in at 6.5 to 7 on the Mohs scale of mineral hardness, it can’t be scratched by softer materials. Use a copper penny or a steel knife and scratch the surface of your stone. If the object leaves a scratch on the stone, it isn’t real black onyx. On 1stDibs, you’ll find a collection of genuine black onyx jewelry from some of the world’s top sellers.