Onyx: The Complete Guide

Black onyx is one of the most commercially important opaque gems in fine jewellery. Its rich, deep black — whether in the form of signet rings, mourning jewellery, gentleman’s accessories, or contemporary statement pieces — has a timeless commercial appeal that has made it a retail staple for centuries. Yet onyx is widely misunderstood: most commercial “black onyx” is dyed chalcedony rather than natural banded agate, and natural black onyx is considerably rarer than the ubiquitous commercial product suggests. Understanding the distinction allows professionals to make accurate representations and educated sourcing decisions.

What Onyx Is — and Is Not

True onyx is a variety of chalcedony (microcrystalline quartz) with parallel banding of black and white layers. In traditional gemological definition, onyx refers specifically to this banded variety. However, in commercial practice, “black onyx” is used almost universally to describe solid black chalcedony, whether naturally black or (far more commonly) dyed.

Natural solid black chalcedony does exist but is relatively rare. The vast majority of commercial black onyx is produced by treating grey or brownish chalcedony with sugar solution and acid — a well-established process that creates a stable black coloration that has been used since Roman times. This treatment is so universal and so accepted in the trade that it is rarely disclosed in retail contexts, though strictly speaking it is a treatment that should be mentioned. The FTC guidelines require disclosure of all treatments that materially affect value.

Historical Significance

Black onyx has been used in signet rings, cameos, and mourning jewellery since antiquity. Roman soldiers wore black onyx intaglio rings as protective amulets. The Victorian mourning jewellery tradition used black onyx extensively following the death of Prince Albert in 1861 and Queen Victoria’s subsequent decades of mourning — this association between black onyx and grief has persisted in Western culture and still influences how some clients perceive the stone.

More positively, black onyx has been central to men’s jewellery and accessories for centuries: cufflinks, tie pins, signet rings, and watch bezels frequently feature black onyx. In contemporary fine jewellery, black onyx is used as a foil for diamonds and coloured stones, providing a dramatic dark background that makes surrounding gems appear to glow.

Green Onyx and Other Varieties

Green onyx is typically dyed chalcedony in green — not a natural variety. Red onyx (sardonyx) involves genuine red-brown banding from iron. Blue onyx in various commercial grades is almost always dyed. The widespread use of dyed chalcedony under the “onyx” name in various colours is a category where professional accuracy matters: each variety should be properly identified and treatment disclosed.

Commercial Applications

Black onyx is versatile across multiple jewellery categories and client demographics. For gentleman’s jewellery (cufflinks, signet rings, tie bars), it remains a classic and commercially reliable choice. For women’s fashion jewellery and statement pieces, black onyx provides bold contrast and dramatic visual impact. For contemporary fine jewellery with pavé diamonds, black onyx backgrounds create a sophisticated graphic quality favoured by designers from Cartier to smaller artisan studios.