Black against gold is one of the few combinations that has never once looked dated. Black Onyx Ring, 18K gold, 1962
Onyx is a variety of chalcedony, which is quartz in microscopically fine form, and strictly speaking the word means banded chalcedony with straight parallel layers. The solid black stone everybody calls onyx is the black layer of that material, and here is the fact that surprises people: nearly all of it has been dyed, and it has been dyed since Roman times. This is not a modern deception. It is one of the oldest continuous techniques in the gem trade.
The family, briefly
| Name | What it is |
|---|---|
| Onyx | Banded chalcedony with straight parallel layers, commonly black and white |
| Black onyx | The black layer, almost always colour-enhanced |
| Sardonyx | Onyx with reddish-brown sard layers instead of black |
| Agate | The same material with curved or irregular banding |
| Carnelian | Translucent orange-red chalcedony |
| Bloodstone | Dark green chalcedony with red spots |
The straight banding is exactly why onyx and sardonyx were the classical material for cameo carving: a carver cuts down through a white layer into a black one and gets a white figure on a dark ground without painting anything. The technique and its opposite are covered in what is a cameo.
The dyeing, and why it is not a problem
The traditional method soaks the stone in a sugar solution, then in sulphuric acid, which carbonises the sugar inside the pores and turns the stone permanently black. Pliny described a version of it in the first century. The colour is stable, permanent, goes right through the material, and does not fade in sunlight or wash out.
So when a dealer says a black onyx is dyed, that is a statement of ordinary fact rather than a confession. What matters much more is whether the stone is chalcedony at all, because the genuine substitute problem is glass.
Onyx against black glass
Black glass, sometimes sold as French jet, was used extensively in Victorian mourning jewelry as a cheaper substitute for jet and onyx, and it turns up constantly.
Temperature. Onyx is cool at first touch and stays cool longer. Glass warms faster.
Bubbles. Under a loupe, glass frequently shows round gas bubbles. Chalcedony never does.
Mould seams. Pressed glass often shows a faint seam at the edge or around the back.
Chips. Glass chips with a curved, shell-like fracture and a very sharp edge. Onyx chips more bluntly.
Weight. Onyx is slightly denser and feels more substantial than glass of the same size.
Jet, vulcanite and bog oak are the other black materials in this territory, all lighter than both onyx and glass, and all warm to the touch immediately.
Eleven grams of gold around a flat black face, which is the classic signet formula. Antique Onyx Signet Ring, 11 grams, 18K gold
Where it appears in antique jewelry
Onyx has three strong periods. Victorian mourning, where black against gold was the required palette and onyx sat alongside jet and black enamel. Signet rings, across the whole nineteenth and twentieth centuries, where a flat black face takes an engraved monogram or crest beautifully, as discussed in what is a signet ring. And Art Deco, where black onyx against diamonds and platinum became the defining contrast of the 1920s, and where the material stopped being about grief and became about geometry.
Care
At 6.5 to 7 on the hardness scale onyx is durable enough for daily wear, but it is porous, which is the thing to remember. Avoid soaking, avoid ultrasonic cleaners, and keep it away from oils, solvents and household chemicals, all of which can penetrate and change the colour over time. Warm water, mild soap, a soft cloth, dried properly. Onyx also dislikes sudden temperature changes, so no steam cleaning.
Frequently asked questions
Is black onyx dyed?
Almost always, and it has been for two thousand years. The traditional sugar and acid method produces a permanent colour that goes through the stone.
Is onyx a real gemstone?
Yes. It is a banded variety of chalcedony, which is microcrystalline quartz, and it sits at 6.5 to 7 on the hardness scale.
How can I tell onyx from black glass?
Check for gas bubbles and mould seams under a loupe, feel the temperature, since glass warms faster, and look at how any chip has fractured.
What is the difference between onyx and agate?
Both are chalcedony. Onyx has straight parallel banding; agate has curved or irregular banding.
Does onyx fade?
No. The traditional dye is permanent and stable in sunlight. Dullness is nearly always surface wear or residue rather than fading.
Can I wear an onyx ring every day?
Yes, with normal care. It is hard enough for daily wear but porous, so keep it away from oils, solvents and prolonged soaking.
What is sardonyx?
Onyx with reddish-brown sard layers rather than black ones. It was the favourite material for classical cameo carving.
Read next: What is a signet ring? · What is mourning jewelry? · What is Art Deco jewelry?
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