* chalcedony gemstone
Onyx: identification, value, and how to spot a fake
Strictly, onyx is banded black-and-white chalcedony. Most market 'onyx' is solid black dyed chalcedony. Used in cameos and intaglios historically.
Published May 30, 2026
Quick facts
- Mineral family
- chalcedony
- Chemical formula
- SiO2 (cryptocrystalline)
- Mohs hardness
- 6.5–7
- Refractive index
- 1.53–1.539
- Specific gravity
- 2.58–2.64
- Crystal system
- trigonal (cryptocrystalline)
- Luster
- vitreous to waxy
Colors
Onyx is found in black, black with white bands. The body color, its saturation, and how evenly it spreads through the stone are among the strongest drivers of value.
How to tell real from imitation
No single test settles it, but a few grounded checks quickly narrow things down for Onyx. Hardness, brilliance, and the list of stones it is commonly confused with all give you something concrete to look for.
- Scratch resistance: genuine Onyx sits at Mohs 6.5–7, so it should resist scratching from softer materials — a stone that mars far too easily is suspect.
- Brilliance & luster: Onyx shows a vitreous to waxy luster and a refractive index of 1.53–1.539; imitations often look glassier or flatter under the same light.
- Density: a real Onyx has a specific gravity near 2.58–2.64, so it feels heavier or lighter in the hand than many look-alikes of the same size.
- Commonly imitated by: black glass, obsidian. If a deal looks too good, suspect one of these substitutes first.
- Fast first read: photograph the stone in the Jewelry Identifier app to estimate the gemstone and its likely value range, then confirm anything important with a gemological lab.
Common treatments
- dye (most jet-black onyx is dyed chalcedony)
Treatments are routine for many Onyx stones, but they materially affect value and should always be disclosed — an untreated, lab-certified stone commands a clear premium over a treated one.
What to know about Onyx
Strictly, onyx is banded black-and-white chalcedony. Most market 'onyx' is solid black dyed chalcedony. Used in cameos and intaglios historically.
Sources
* Frequently asked
FAQ
- Q. Is Onyx hard enough for everyday rings?
- A. Not ideally. At a Mohs hardness of 6.5–7, Onyx is soft enough to scratch with everyday wear, so it is better suited to earrings, pendants, or occasional-wear rings with protective settings.
- Q. How can I tell Onyx from black glass?
- A. Onyx and black glass can look alike to the naked eye, but they differ in measurable properties — refractive index (1.53–1.539 for Onyx), specific gravity (2.58–2.64), and hardness (Mohs 6.5–7). A gemologist confirms the distinction with a refractometer and loupe; the Jewelry Identifier app gives a fast first read from a photo.
- Q. Does Onyx come in colors other than black?
- A. Yes. Onyx occurs in black, black with white bands. Color is driven by trace elements and treatment history, and it is one of the biggest factors in a stone's value.
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