Onyx is a form of chalcedony, which is a microcrystalline quartz. It is formed in the gas cavities of lava. Its multi-layering is similar to that of agates. When the stone has a black base and a white upper layer, it's called onyx. (Black onyx is uniformly black chalcedony.) When the base is brown, it is called Sard-onyx. And carnelian-onyx has a red base. Much of what is referred to as "black onyx" is gray chalcedony that has been chemically treated to turn it into a rich black. The grey stone can be saturated with a solution of cobalt chlorate and chlorammonium, or it can be immersed for a prolonged period in a sugar solution and then heated in sulphuric acid to carbonize the sugar.