Onyx vs Obsidian
Onyx and Obsidian are easy to confuse. Here’s how they stack up on the properties that actually separate them.
| Onyx | Obsidian | |
|---|---|---|
| Also known as | Chalcedony (Chalcedony family) | — |
| Colour | Black | Black |
| Mohs hardness | 6.5–7 | 5–6 |
| Lustre | Vitreous, silky | Vitreous |
| Streak | White | — |
| Crystal system | Trigonal (quartz), Monoclinic (moganite) | — |
| Transparency | Translucent | — |
| Cleavage | None | — |
| Fracture | Uneven, conchoidal | Conchoidal |
| Chemical formula | SiO 2 (silicon dioxide) | — |
| Specific gravity | 2.55–2.70 | c. 2.4 |
| Typical value | — | $5–$9 a polished piece |
Value = estimate only (gems priced per carat from price guides; common stones from recent eBay sold listings). See each stone's page and our methodology.
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Frequently asked questions
What's the easiest way to tell Onyx from Obsidian?
Hardness is the quickest test: Onyx is Mohs 6.5–7 and Obsidian is Mohs 5–6.