oxides
Onyx
SiO2
- Also known as
- black onyxsardonyx
- Crystal system
- trigonal
- Hardness
- 6.5–7 on the Mohs scale
- Lustre
- waxy
- Colour
- Black, or black and white in straight parallel bands
- Where it forms
- As agate, in volcanic cavities, but deposited in flat layers under gravity rather than following the cavity wall.
- Chiefly from
- BrazilIndiaUruguay
Why it is that colour
True onyx is banded chalcedony where the bands are flat rather than concentric. The uniform black of nearly all commercial onyx is not natural: it is grey chalcedony soaked in sugar solution and then in sulfuric acid, which carbonises the sugar in the pores.
Safety
No hazard from handling. The treatment is permanent and leaves nothing mobile in the stone.
Where it comes from
Almost universally dyed. The sugar-and-acid method was developed in Germany in the early 1800s and is now standard, cheap and rarely disclosed. Genuinely natural solid black chalcedony is uncommon.
Historical use
The great classical cameos are onyx and sardonyx — the Gemma Augustea and the Great Cameo of France among them — exploiting the layering to carve portraits in relief. The technique depends entirely on the bands being flat and sharply divided.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Onyx is discussed at length in Greek and Roman sources and was the material of choice for cameos, because the flat bands let a carver cut a white figure against a dark ground. Ancient associations are mixed and sometimes negative — some Roman sources treat it as a stone of discord. The modern reading as a protective grounding stone is New Age.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
Two quite different materials are sold as onyx and it is worth keeping them apart. This entry is chalcedony, hardness around 7. The translucent honey-and-cream banded stone used for lamps, bathroom fittings and decorative bowls is "onyx marble", a banded calcite, hardness 3, which will scratch with a coin and etch with lemon juice. They are unrelated.
The cameo tradition is the reason onyx mattered. If you want to carve a white face on a black ground, you need a stone with sharply bounded parallel layers of contrasting colour and no cleavage — and there is essentially one material that does that.
Identity checked against IMA-CNMNC master list, June 2025.