Gemstones & cuts
Onyx, black chalcedony and the mourning trade
The graphic black of Art Deco and the discretion of Victorian mourning.
Written by our in-house jewelry team · Updated August 1, 2026 · 5 min read
In short
Onyx is a banded variety of chalcedony, a microcrystalline quartz, about 7 on the Mohs scale. Nearly all commercial black onyx is dyed, and has been for well over a century, which is standard and accepted practice. It is the stone that gives Art Deco jewelry its hard black-and-white contrast.
Onyx does something no gemstone does: it absorbs light instead of returning it. That is precisely why Deco designers set it against diamond.
Shop the pieces: Onyx, curated and available now.
What onyx is, strictly speaking
A banded chalcedony that the trade has long used as a name for any solid black chalcedony, dyed or not.
Species and group: Chalcedony (microcrystalline quartz). Mohs hardness: 7. Typical colors: Black, black and white banded, and in sardonyx, brown and white.
- Even, opaque color with a glassy polish and no visible crystal structure
- Takes carving and inlay beautifully, hence cameos and intaglios in sardonyx
- Slightly warmer to the touch than glass and noticeably heavier than plastic imitations
- Chips at edges rather than fracturing through
Dyeing is the norm here
Sugar-and-acid dyeing of grey chalcedony to a permanent black goes back to antiquity and was industrial practice by the nineteenth century. Nobody in the period considered it deceptive, and neither do we, as long as it is stated.
Our gemologist examines every stone before a piece is listed, and the description states what is known and what cannot be determined without a laboratory report.
- Dyeing to a uniform black: standard for well over a century and considered permanent
- Sugar-acid treatment, an old dyeing method still described in gemological literature
- Black glass and plastic imitations appear in costume pieces and are identified by weight, temperature and mold lines
- Composite or reconstituted material exists in modern pieces and is disclosed when present
Onyx, glass or plastic
Black is easy to fake, and mourning jewelry attracted cheap substitutes from the start. Temperature, weight and edges give it away.
- Glass imitations show bubbles or mold seams and feel cold and light
- Real onyx shows conchoidal chipping at damaged edges, glass shows a shell-like fracture with sharper edges
- Look for tiny surface pits from polishing, common in genuine chalcedony
- Sardonyx cameos show parallel color bands used deliberately by the carver
From mourning brooches to Deco contrast
Victorians used onyx and jet for half-mourning jewelry, where black was required but ostentation was not. Art Deco jewelers then rediscovered it as pure graphic contrast against platinum and diamond.
It remains one of the most wearable antique materials for men's jewelry: signet rings, cufflinks and dress sets.
Frequently asked questions
- Is dyed onyx a problem?
- No. Almost all black onyx is dyed and has been since the nineteenth century. It is stable, standard and disclosed.
- What is the difference between onyx and jet?
- Jet is fossilized wood, much lighter and warmer to the touch. Onyx is a quartz and feels dense and cool.
- Can onyx be repolished?
- Yes, a lapidary can repolish a scratched onyx panel, though a badly chipped one usually needs recutting or replacement.
Written by our estate jewelry team and reviewed by our in-house gemologist. We have been buying and selling European antique jewelry since 1973.
From the reading to the piece
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Each piece is one of a kind and sells once, so this collection changes constantly.
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