Comparison · Igneous Rocks
Dunite vs Rainbow Obsidian
Dunite and Rainbow Obsidian are easy to confuse because both are igneous rocks. Dunite is harder (Mohs 6.5–7) than Rainbow Obsidian (Mohs 5.5), so Dunite will scratch Rainbow 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 Rainbow Obsidian
- Dunite is harder (Mohs 6.5–7) than Rainbow Obsidian (Mohs 5.5), so Dunite will scratch Rainbow 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 Rainbow Obsidian).
- Hold a thin edge to the light: Dunite is Opaque (translucent in thin section), Rainbow Obsidian is opaque.
- They overlap in green and yellow, but Dunite also comes in brown and Rainbow Obsidian also comes in black, multicolored, and purple.
Dunite vs Rainbow Obsidian properties
| Property | Dunite | Rainbow 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) | opaque |
| Specific gravity | 3.2–3.3 | 2.4 |
| Crystal system | — | amorphous |
| Common colors | Olive green, Yellowish green, Dark green, Brown (weathered) | Black with rainbow sheen, Purple, Green, Gold |
| Rarity | common | uncommon |
| Typical specimen value | $1–$75+ | $10–$200+ |
Which is more valuable?
Rainbow Obsidian is usually the more valuable of the two: a typical collector specimen runs about $50 against $10 for Dunite, and premium Rainbow Obsidian can reach $200 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 Rainbow Obsidian
Rainbow obsidian is a type of volcanic glass that displays bands of colorful iridescence when polished and viewed in bright light.
Dunite vs Rainbow Obsidian FAQ
Is Dunite the same as Rainbow 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 Rainbow Obsidian (Mohs 5.5), so Dunite will scratch Rainbow 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 Rainbow Obsidian?
Dunite is harder (Mohs 6.5–7) than Rainbow Obsidian (Mohs 5.5), so Dunite will scratch Rainbow 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 Rainbow Obsidian). Hold a thin edge to the light: Dunite is Opaque (translucent in thin section), Rainbow Obsidian is opaque.
Which is harder, Dunite or Rainbow Obsidian?
Dunite is harder: Mohs 6.5–7 against Mohs 5.5 for Rainbow Obsidian.
Which is more valuable, Dunite or Rainbow Obsidian?
Rainbow Obsidian is usually the more valuable of the two: a typical collector specimen runs about $50 against $10 for Dunite, and premium Rainbow Obsidian can reach $200 or more.
