Black Onyx

Here's a fact that surprises most people: virtually all black onyx is dyed. Natural solid black onyx is extremely rare in nature. The overwhelming majority of 'black onyx' in jewelry and the crystal market is gray or pale agate that has been dyed black using a sugar-acid treatment that has been practiced for thousands of years.

The traditional process involves soaking pale agate in a sugar solution for weeks, then treating it with sulfuric acid. The acid carbonizes the sugar that has penetrated the stone's microscopic pores, turning it permanently black. This treatment is ancient (practiced by the Romans), permanent, and universally accepted in the gem trade - but almost never disclosed to consumers.

True onyx, mineralogically speaking, is banded chalcedony with alternating black and white layers. 'Sardonyx' has brown/red and white bands. The term 'onyx' has evolved in the marketplace to mean solid black chalcedony regardless of whether it's natural or treated.

Black onyx is identified as solid black chalcedony at hardness 7. In practice, identification means determining whether it's natural or dyed - and it's almost always dyed.

Distinguish from obsidian (volcanic glass, conchoidal fracture, slightly translucent at thin edges), black tourmaline (crystal faces, striations), jet (organic, much lighter weight), and black spinel (much denser, vitreous rather than waxy). Under strong transmitted light, dyed onyx may show the original banding of the agate that was dyed.