Gem library
Onyx
The black chalcedony of cameos
Onyx is chalcedony with straight parallel bands, most often black or black and white. An essential market fact: nearly all black onyx sold today is dyed grey agate, following a sugar-and-acid recipe more than two thousand years old.
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.55 – 2.70
- Crystal system
- Trigonal
- Refractive index
- 1.530 – 1.540
- Cleavage
- None
Mineralogical identity
- Family
- Microcrystalline quartz — chalcedony
- Species
- Chalcedony
- Variety
- Onyx (straight-banded chalcedony)
- Chemical formula
- SiO₂
- Crystal system
- Trigonal (microcrystalline aggregate)
- Classification
- Semi-precious stone
- Rarity
- Very common
- Indicative price
- €0.30 – 4/ct
Etymology
From the Greek onux, «nail» or «claw»: legend has it that Eros cut the sleeping Venus's nails and the Fates turned them to stone.
Physical & optical properties
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.55 – 2.70
- Refractive index
- 1.530 – 1.540
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Vitreous to waxy
- Diaphaneity
- Opaque, slightly translucent at edges
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- black, black and white
Formation, geology & history
A black that has been manufactured since antiquity
Pliny the Elder already described the process in the first century: soak a porous agate in honey or hot sugar, then in sulphuric acid. The sugar penetrates the pores and the acid carbonises it into a deep, perfectly stable black. Idar-Oberstein industrialised the method in the nineteenth century. The treatment is irreversible, does not fade, and is not considered fraud: the trade accepts it universally, provided it is declared. Natural black onyx exists, but it is rare and often less even.
Onyx and cameos
The real value of onyx in glyptics lies in its bands: a carver working a black plate with a superficial white layer can release a white motif in relief on a black ground. That is the principle of the cameo, perfected by Hellenistic then Roman workshops — the Gemma Augustea and the Grand Camée de France are cut from multi-layered sardonyx. The cameo workshops of Torre del Greco, near Naples, continue the technique, though they mostly work conch shell, which is softer. Sardonyx combines brown and white bands, onyx proper black and white.
The great onyx misunderstanding
The word is also used, wrongly, for an entirely different material: «onyx marble» or Mexican onyx, which is in fact banded calcite or aragonite — translucent, green, honey or white. It is made into lamps, trays and washbasins. Its hardness is only 3: a coin scratches it and vinegar dissolves it. Confusing the two is the most frequent error in the decorative stone trade: if your «onyx» is scratched by a knife, it is calcite.
How to recognise it
- Deep black, waxy lustrePolished, onyx has a dense, slightly waxy black with no metallic sheen. Thin edges are sometimes very faintly translucent.
- Hardness 7It scratches glass. An «onyx» scratched by a knife is banded calcite (onyx marble), not chalcedony.
- No bubbles or seamsBlack glass, heavily used as an imitation, shows bubbles and mould lines and warms quickly in the hand.
Imitations, treatments & confusions
- Black glassEverywhere in cheap beads and intaglios. Bubbles under the loupe, visible moulds, a slightly greasy lustre, warms fast.
- Banded calcite or aragonite (onyx marble)Sold as onyx for decoration. Translucent, veined honey or green, hardness 3, effervescent in vinegar.
- Filled black resinLight, warm, scratched by a needle, plastic smell on the hot point. Common in costume jewellery.
Deposits & origins
Brazil, Uruguay, India, Madagascar; dyeing is carried out mainly in Germany (Idar-Oberstein) and India. Ancient banded onyx came from Arabia and India
Care & precautions
- Warm soapy water and a soft brush: black-dyed chalcedony is perfectly stable.
- Avoid solvents and acetone, which can attack the finishing waxes sometimes applied.
- Prolonged salt is not advisable on porous or antique pieces.
- Ultrasonic-safe on sound stones; avoid on antique cameos and glued pieces.
Traditions & crystal healing
- Chakras
- Root
- Zodiac signs
- Capricorn · Scorpio
- Element
- Earth
Tradition makes it a stone of protection and firmness: it is said to help you hold your ground, set limits and get through bereavement. That funerary use is old — onyx was long the stone of half-mourning in Victorian jewellery, alongside jet.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is my black onyx dyed?
Almost certainly yes, and that is not a flaw: the sugar-and-acid dye is an ancient, stable and universally accepted process. Homogeneous natural black onyx is very rare.
Onyx or obsidian?
Onyx is a chalcedony (hardness 7, conchoidal fracture, waxy lustre); obsidian is a volcanic glass (hardness 5, very sharp fracture, almost greasy vitreous lustre). Obsidian scratches more easily.
What is Mexican onyx?
Banded calcite or aragonite, translucent and soft (hardness 3), used in decoration. Despite the trade name it has no mineralogical relation to true onyx.