Comparison · Minerals vs Semi-Precious Stones
Chalcedony vs Opal
Chalcedony and Opal are easy to confuse. Chalcedony is harder (Mohs 6.5–7) than Opal (Mohs 5.5), so Chalcedony will scratch Opal and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Chalcedony and Opal
- Chalcedony is harder (Mohs 6.5–7) than Opal (Mohs 5.5), so Chalcedony will scratch Opal and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
- Chalcedony feels noticeably heavier for its size (specific gravity 2.6–2.64 versus 2.1 for Opal).
- Hold a thin edge to the light: Chalcedony is Translucent, Opal is translucent.
- They overlap in blue and white, but Chalcedony also comes in gray, brown, and multicolored and Opal also comes in black and pink.
- Well-formed crystals give it away: Chalcedony is Trigonal (cryptocrystalline), Opal is amorphous.
Chalcedony vs Opal properties
| Property | Chalcedony | Opal |
|---|---|---|
| Type | Minerals | Semi-Precious Stones |
| Mohs hardness | 6.5–7 | 5.5 |
| Luster | Waxy to vitreous | vitreous |
| Streak | White | White |
| Transparency | Translucent | translucent |
| Specific gravity | 2.6–2.64 | 2.1 |
| Crystal system | Trigonal (cryptocrystalline) | amorphous |
| Common colors | Blue-gray, White, Brown, Various (often dyed) | White, Black, Fire, Blue, Pink |
| Rarity | common | uncommon |
| Typical specimen value | $2–$150+ | $50–$10000+ |
Which is more valuable?
Opal is usually the more valuable of the two: a typical collector specimen runs about $500 against $20 for Chalcedony, and premium Opal can reach $10000 or more.
About Chalcedony
Chalcedony is a cryptocrystalline form of quartz made of microscopic fibrous crystals too small to resolve without magnification, chemically identical to ordinary quartz but structurally distinct from it. It serves as the parent material for a large family of banded and colored varieties, including agate, onyx, carnelian, and chrysoprase. Chalcedony typically forms a waxy, translucent to semi-translucent mass rather than distinct visible crystals.
About Opal
Opal is famous for its play-of-color, caused by the diffraction of light through microscopic silica spheres. Australia produces 95% of the world's opals.
Chalcedony vs Opal FAQ
Is Chalcedony the same as Opal?
No. Chalcedony is a mineral while Opal is a semi-precious stone. Chalcedony is harder (Mohs 6.5–7) than Opal (Mohs 5.5), so Chalcedony will scratch Opal and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
How can you tell Chalcedony from Opal?
Chalcedony is harder (Mohs 6.5–7) than Opal (Mohs 5.5), so Chalcedony will scratch Opal and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Chalcedony feels noticeably heavier for its size (specific gravity 2.6–2.64 versus 2.1 for Opal). Hold a thin edge to the light: Chalcedony is Translucent, Opal is translucent.
Which is harder, Chalcedony or Opal?
Chalcedony is harder: Mohs 6.5–7 against Mohs 5.5 for Opal.
Which is more valuable, Chalcedony or Opal?
Opal is usually the more valuable of the two: a typical collector specimen runs about $500 against $20 for Chalcedony, and premium Opal can reach $10000 or more.
