Comparison · Igneous Rocks
Dunite vs Obsidian
Dunite and Obsidian are easy to confuse because both are igneous rocks. Dunite is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Dunite will scratch Obsidian 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 Dunite and Obsidian
- Dunite is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Dunite will scratch Obsidian 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.
- Dunite feels noticeably heavier for its size (specific gravity 3.2–3.3 versus 2.4 for Obsidian).
- Hold a thin edge to the light: Dunite is Opaque (translucent in thin section), Obsidian is translucent.
- They overlap in green and brown, but Dunite also comes in yellow and Obsidian also comes in black and multicolored.
Dunite vs Obsidian properties
| Property | Dunite | Obsidian |
|---|---|---|
| Type | Igneous Rocks | Igneous Rocks |
| Mohs hardness | 6.5–7 | 5.5 |
| Luster | Vitreous (olivine grains) | vitreous |
| Streak | — | White |
| Transparency | Opaque (translucent in thin section) | translucent |
| Specific gravity | 3.2–3.3 | 2.4 |
| Crystal system | — | amorphous |
| Common colors | Olive green, Yellowish green, Dark green, Brown (weathered) | Black, Brown, Green, Rainbow |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $5–$100+ |
Which is more valuable?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Dunite, and premium Obsidian can reach $100 or more.
About Dunite
Dunite is a coarse-grained ultramafic igneous rock composed almost entirely of olivine, typically more than 90 percent, with minor chromite, pyroxene, or spinel. It represents mantle material or cumulate layers formed by the settling of olivine crystals in a magma chamber, making it one of the most direct samples of Earth's upper mantle available at the surface. Its name comes from Dun Mountain in New Zealand, where it was first described.
About Obsidian
Obsidian is a naturally occurring volcanic glass formed from rapidly cooled lava. Its sharp edges made it invaluable for ancient tools and weapons.
Dunite vs Obsidian FAQ
Is Dunite the same as Obsidian?
No. Both are igneous rocks, but they are distinct rock types with different hardness, streak or luster. Dunite is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Dunite will scratch Obsidian 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 Dunite from Obsidian?
Dunite is harder (Mohs 6.5–7) than Obsidian (Mohs 5.5), so Dunite will scratch Obsidian 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. Dunite feels noticeably heavier for its size (specific gravity 3.2–3.3 versus 2.4 for Obsidian). Hold a thin edge to the light: Dunite is Opaque (translucent in thin section), Obsidian is translucent.
Which is harder, Dunite or Obsidian?
Dunite is harder: Mohs 6.5–7 against Mohs 5.5 for Obsidian.
Which is more valuable, Dunite or Obsidian?
Obsidian is usually the more valuable of the two: a typical collector specimen runs about $20 against $10 for Dunite, and premium Obsidian can reach $100 or more.
