Onyx

One name, two familiar appearances

In the classic gemmological sense, onyx is layered chalcedony. It is often described as black with white bands. Shops commonly use “black onyx” for chalcedony that is black all over. The two meanings differ. A modern pendant with an unbroken black face need not have been a uniform black in nature.

Onyx belongs with agate and other fine-grained quartz materials. It is not the carbonate decorative stone also sold with a name containing “onyx.” A large vessel or building slab may bear that label and let light through. Check what mineral it is before you use care advice meant for quartz.

Chalcedony usually has Mohs hardness about 6.5–7. This guides us on scratch resistance. It does not promise that a piece cannot chip. Cracks, inclusions and treatments can all affect how a specific object withstands wear.

Evidence: GIA: Onyx and anniversary gemstones, GIA: August Birthstones, sardonyx care, Getty Research Institute: onyx marble, Art & Architecture Thesaurus

Colour and the treatment question

GIA notes that naturally black chalcedony is uncommon. Much black chalcedony sold for jewellery has been treated. Treatment changes its look and should be disclosed. It does not by itself turn the material into an imitation. A different black material used in place of onyx is a separate case.

Ask for a plain description. Dyed black chalcedony is useful wording when that is what is known. “Genuine onyx” alone does not say whether its colour is natural. A photograph or weight in the hand cannot replace identification. Neither can a scratch test at home. Such a test may damage a finished object.

Evidence: GIA: Onyx and anniversary gemstones, GIA: An Introduction to Gem Treatments

Carving across layers

A cameo has an image raised above the ground around it. A carver can use a pale layer for the figure and a darker layer behind it. Intaglio does the reverse: the image is cut into the surface. These are ways to carve, not names for different minerals.

The British Museum records an onyx cameo from the first to second century CE. It shows a draped woman placing an object on an altar. This small carving shows the long link between onyx and fine engraving. Its date belongs to that particular object. It does not date the discovery of onyx or the start of all cameo making.

Evidence: British Museum: Roman onyx cameo, 1889,0517.3, GIA: August Birthstones, sardonyx care

Use, origins and care

GIA discusses sardonyx, a related layered material. Its known sources include India, Brazil and Uruguay. These records give us context for the supply of chalcedony. They do not prove where every black onyx jewel came from.

For a sound modern piece, mild soap, lukewarm water and a soft brush are cautious choices. Avoid solvents and long exposure to strong sunlight if colour treatment is known or uncertain. Thin carved details in a cameo need protection from blows. Store it apart from harder stones. Seek professional advice before using steam, ultrasonic cleaning or heat in a repair.

Evidence: GIA: August Birthstones, sardonyx care, GIA: An Introduction to Gem Treatments, GIA: Gemstone Durability, Design to Display

Layer geometry becomes part of a design

Classic onyx has straight layers that run broadly parallel to each other. Their position matters as much as their colour. A carver must check the pale layer's thickness and where the dark layer starts. Cutting through the pale layer in the wrong place may remove a detail needed for the design.

A layered cameo combines a pattern in the rock with a planned image. A plain black plaque instead draws attention to outline and polish. We can admire both, but describe them in different ways. Neither the carving method nor a stylish setting proves that the chalcedony has untreated colour.

Evidence: GIA: Onyx and anniversary gemstones, British Museum: Roman onyx cameo, 1889,0517.3, GIA: August Birthstones, sardonyx care

A museum label and a modern sales label

The British Museum's Roman cameo is only about a centimetre across. Its catalogue date, carved figure and acquisition record describe that object. A modern pendant that looks like it is not the same age. Record any claim of antique origin apart from the material name. Keep a museum's catalogue entry distinct from a seller's comparison that has no supporting records.

Imagine a striped green bowl labelled onyx beside a black chalcedony ring. The Getty thesaurus explains that onyx marble can be calcite. One shared word does not justify one cleaning method. First identify the bowl's material. Then consider its care. Never try a household acid test on a finished carving.

Evidence: British Museum: Roman onyx cameo, 1889,0517.3, Getty Research Institute: onyx marble, Art & Architecture Thesaurus, GIA: Gemstone Durability, Design to Display

Cameo, intaglio and material names

Relief stands above the ground around it. Intaglio is cut into that ground. A cameo uses relief, often with contrasting layers. Sardonyx is layered chalcedony with sard-coloured material and onyx layers. A simulant looks like another material but has a different identity. Treated chalcedony remains chalcedony. Its treatment should still be part of the description.

Evidence: GIA: Onyx and anniversary gemstones, GIA: August Birthstones, sardonyx care, GIA: An Introduction to Gem Treatments

Self-check: three objects called onyx

Must a black ring, a layered cameo and a decorative bowl be the same mineral with the same treatments? No. First find out the actual material. Then check its treatment and condition. Does a carved classical figure prove Roman manufacture? No. Style gives us a place to start research. It cannot replace records that support an attribution.

Evidence: GIA: Onyx and anniversary gemstones, British Museum: Roman onyx cameo, 1889,0517.3, Getty Research Institute: onyx marble, Art & Architecture Thesaurus, GIA: An Introduction to Gem Treatments

Places in the story

These places describe documented sources or historical settings. They do not establish the origin of an individual object.