RocksdayChalcedony vs Opal
How to tell them apart
Quartz made of microscopic fibers. Plain chalcedony is soft blue-gray and often grows in bubbly, grape-like crusts.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
Hydrated silica with no crystal structure. In precious opal, stacked microscopic spheres split light into flashing rainbow patches.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
Key differences
- Opal is softer and lighter, and may show color play.
Side by side
| Property | Chalcedony | Opal |
|---|---|---|
| Type | Mineral | Mineraloid |
| Mohs hardness | 6.5–7 | 5.5–6.5 |
| Luster | Waxy | Vitreous to waxy |
| Streak | White | White |
| Colors | Blue, White, Gray | White, Black, Orange |
| Crystal system / structure | Microcrystalline quartz | Amorphous |
| Specific gravity | 2.58–2.64 | 1.9–2.3 |
| Formula | SiO₂ | SiO₂·nH₂O |
| Found in | Oregon; Namibia | Coober Pedy, Australia; Lightning Ridge, Australia |
Properties that are the same for both are grayed out.
How to identify chalcedony
- Waxy, translucent and milky.
- Bubbly 'botryoidal' surfaces are common.
- No crystal faces; scratches glass.
Crystal system: Microcrystalline quartz
Full Chalcedony guide →How to identify opal
- Flashes of spectral color that shift as you turn it.
- Waxy to glassy luster.
- Light in weight for a gemstone.
- Common opal lacks color play and looks milky.
Structure: Amorphous
Full Opal guide →More comparisons
Could you tell them apart from a blurry close-up?
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