Sounds like a simple question but the way this word is used in the jewelry and decorative arts industry today actually makes it a bit more complicated. (Please also see our section entitled onyx marble.) The onyx used in most jewelry today is actually a banded agate that is a

cryptocrystalline quartz. How come it's not banded in our jewelry you ask? So much onyx is in demand for the jewelry industry that something had to be done to assure a plentiful supply. Low grade agate is placed in a sugar solution at low temperature for 2 to 3 weeks. The stone absorbs as much solution as it can. Once it stops absorbing the solution, it is removed from the bath and cleaned. The agate is then placed in sulfuric acid where it enters the pores and "carbonizes" the sugar turning the agate black. (Some of the layers in agate are impenetrable to the solution, however, and will continue to look gray rather than black once the process is finished.) The result, after cutting and polishing, will be what we know as onyx in the marketplace today.