You’ve probably heard the term “black onyx”, referring to the opaque, black-colored gemstone commonly used in both fine and semi-precious jewelry. There is actually no black onyx gemstone. Rather, this gemstone is technically treated black chalcedony.
Natural black chalcedony is rare, but treated stones are widely available and have been popular for decades. A common method for achieving black chalcedony involves soaking gray chalcedony in a sugar-based solution, followed by immersion in a hot sulfuric acid bath. Carbonization of the sugar results in a permanent black color treatment.
While treated black chalcedony is commercially called “black onyx”, the name is actually a misnomer. Onyx is a variety of chalcedony that contains both black and white bands. Since treated black chalcedony has neither banding, nor any white color, it is not technically an onyx.
Next time you see black onyx jewelry, you’ll know this little tidbit…that there is no such gemstone as black onyx, rather it is really dyed black chalcedony.