Onyx (black, button cut)

"Black onyx" is the trade name for uniformly black chalcedony (microcrystalline quartz, SiO₂). Natural black chalcedony is rare: almost all black onyx on the market is grey or banded chalcedony that has been coloured black, traditionally by soaking it in sugar solution and then in sulfuric acid, which carbonises the sugar in its pores. Strictly, onyx means chalcedony with straight black-and-white bands, so black onyx is a treated natural stone rather than a separate mineral.