Onyx
Chalcedony (quartz)JewelryWiki Editorial·2 sources·Reviewed 2026-08-04
Chalcedony with straight, parallel bands — most often sold as the solid black stone, which is usually dyed. The name causes more confusion than any other in the trade: “black onyx” is ordinary grey chalcedony that has been dyed black, and that is normal practice rather than fraud, provided it is disclosed.
Properties
| Property | Value | Authority |
|---|---|---|
| Mohs hardness | 7 [1] | — |
| Specific gravity | 2.55–2.91 [1] | — |
| Chemical formula | SiO2 Read from the species entry, Chalcedony. A variety shares its species’ formula. [2] | — |
| Crystal system | Hexagonal (trigonal), microcrystalline [1] | — |
| Common treatments | Dyeing, and it is the norm rather than the exception. IGS lists dyeing among chalcedony’s typical treatments, and GIA records chalcedony among the materials both bleached and dyed since ancient times. Assume a solid-black onyx is dyed unless told otherwise. [1] | — |
| Care cautions | Because the colour is usually dye, keep it away from acetone and alcohol, which can dissolve it, and out of prolonged sunlight, which can fade it. [1] | — |
Sources
- International Gem Society — gemstone articles — International Gem Society (IGS, gemstone information articles (gemsociety.org))
- Handbook of Mineralogy — Mineral Data Publishing / Mineralogical Society of America (Handbook of Mineralogy, Mineral Data Publishing)