Comparison · Igneous Rocks
Gabbro vs Rainbow Obsidian
Gabbro and Rainbow Obsidian are easy to confuse because both are igneous rocks. Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Gabbro and Rainbow Obsidian
- Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Gabbro feels noticeably heavier for its size (specific gravity 2.9–3.1 versus 2.4 for Rainbow Obsidian).
- Hold a thin edge to the light: Gabbro is Opaque, Rainbow Obsidian is opaque.
- They overlap in black, green, and multicolored, but Gabbro also comes in gray and white and Rainbow Obsidian also comes in purple and yellow.
Gabbro vs Rainbow Obsidian properties
| Property | Gabbro | Rainbow Obsidian |
|---|---|---|
| Type | Igneous Rocks | Igneous Rocks |
| Mohs hardness | 5–6.5 | 5.5 |
| Luster | Dull to vitreous (in crystals) | vitreous |
| Streak | — | White |
| Transparency | Opaque | opaque |
| Specific gravity | 2.9–3.1 | 2.4 |
| Crystal system | — | amorphous |
| Common colors | Dark gray, Black, Greenish black, Speckled black and white | 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 Gabbro, and premium Rainbow Obsidian can reach $200 or more.
About Gabbro
Gabbro is a coarse-grained, dark-colored intrusive igneous rock composed mainly of calcium-rich plagioclase feldspar and pyroxene, with the plutonic equivalent of basalt. It forms deep underground where mafic magma cools slowly enough for its mineral grains to grow large and interlock visibly. Its dense, speckled black-and-white texture makes it a popular dimension stone sold commercially as black granite.
About Rainbow Obsidian
Rainbow obsidian is a type of volcanic glass that displays bands of colorful iridescence when polished and viewed in bright light.
Gabbro vs Rainbow Obsidian FAQ
Is Gabbro the same as Rainbow Obsidian?
No. Both are igneous rocks, but they are distinct rock types with different hardness, streak or luster. Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Gabbro from Rainbow Obsidian?
Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gabbro feels noticeably heavier for its size (specific gravity 2.9–3.1 versus 2.4 for Rainbow Obsidian). Hold a thin edge to the light: Gabbro is Opaque, Rainbow Obsidian is opaque.
Which is harder, Gabbro or Rainbow Obsidian?
They are about equally hard, both near Mohs 5–6.5.
Which is more valuable, Gabbro or Rainbow Obsidian?
Rainbow Obsidian is usually the more valuable of the two: a typical collector specimen runs about $50 against $10 for Gabbro, and premium Rainbow Obsidian can reach $200 or more.
