The term "black onyx" is often used for which type of stone?

The term "black onyx" is most accurately associated with treated black chalcedony. Chalcedony is a form of cryptocrystalline silica, and natural chalcedony can vary in color. When it comes to black onyx, what is typically encountered in the market is often treated black chalcedony. This treatment can involve processes to enhance the black color or achieve a uniform appearance.

Natural black agate, while similar, may not be as commonly marketed as onyx and can show more banding or different characteristics than what is typically identified as onyx. Black tourmaline is a distinct mineral, which doesn’t share the same properties or formation as onyx, and black spinel, though it can be black, is not typically categorized as onyx. The use of the term "black onyx" in most jewelry contexts refers to the more commonly treated version of chalcedony, which is why it is correct to associate it with treated black chalcedony.