Comparison · Igneous Rocks
Basalt vs Gabbro
Basalt and Gabbro are easy to confuse because both are igneous rocks. Both sit near Mohs 6, 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 Basalt and Gabbro
- Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Luster is different: Basalt is dull while Gabbro is dull to vitreous (in crystals).
- Hold a thin edge to the light: Basalt is opaque, Gabbro is Opaque.
- They overlap in black, gray, and green, but Gabbro also comes in white and multicolored.
Basalt vs Gabbro properties
| Property | Basalt | Gabbro |
|---|---|---|
| Type | Igneous Rocks | Igneous Rocks |
| Mohs hardness | 6 | 5–6.5 |
| Luster | dull | Dull to vitreous (in crystals) |
| Streak | Variable; test individual minerals | — |
| Transparency | opaque | Opaque |
| Specific gravity | 3 | 2.9–3.1 |
| Crystal system | varies | — |
| Common colors | Black, Dark Gray, Dark Green | Dark gray, Black, Greenish black, Speckled black and white |
| Rarity | common | common |
| Typical specimen value | $1–$30+ | $1–$75+ |
Which is more valuable?
Gabbro is usually the more valuable of the two: a typical collector specimen runs about $10 against $5 for Basalt, and premium Gabbro can reach $75 or more.
About Basalt
Basalt is a dark, fine-grained volcanic rock that makes up most of the ocean floor and Hawaiian Islands. It often forms distinctive columnar jointing.
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.
Basalt vs Gabbro FAQ
Is Basalt the same as Gabbro?
No. Both are igneous rocks, but they are distinct rock types with different hardness, streak or luster. Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Basalt from Gabbro?
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Luster is different: Basalt is dull while Gabbro is dull to vitreous (in crystals). Hold a thin edge to the light: Basalt is opaque, Gabbro is Opaque.
Which is harder, Basalt or Gabbro?
They are about equally hard, both near Mohs 6.
Which is more valuable, Basalt or Gabbro?
Gabbro is usually the more valuable of the two: a typical collector specimen runs about $10 against $5 for Basalt, and premium Gabbro can reach $75 or more.
