RocksdayDiorite vs Gabbro
How to tell them apart
A 'salt-and-pepper' coarse-grained rock of white plagioclase and black hornblende, between granite and gabbro in composition.
Photo: Michael C. Rygel, CC BY-SA 3.0
A dark, coarse-grained rock made of pyroxene and plagioclase — basalt that cooled slowly underground.
Photo: Darla Sondrol, CC0
Key differences
- Gabbro is much darker overall.
- Diorite has much more white, giving a salt-and-pepper look.
Side by side
| Property | Diorite | Gabbro |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 6–6.5 |
| Luster | Granular | Granular |
| Colors | White, Black, Gray | Black, Gray, Green |
| Crystal system / structure | Coarse-grained (phaneritic) | Coarse-grained (phaneritic) |
| Specific gravity | 2.8–3 | 2.9–3.1 |
| Found in | Guernsey, Channel Islands; Sierra Nevada, California | Duluth Complex, Minnesota; Cuillin, Isle of Skye |
Properties that are the same for both are grayed out.
How to identify diorite
- Roughly equal white and black speckles.
- Coarse grains visible by eye.
- Little or no glassy quartz, no pink feldspar.
- Hard and heavy.
Structure: Coarse-grained (phaneritic)
Full Diorite guide →How to identify gabbro
- Dark gray to black with visible crystals.
- Heavy.
- Little or no quartz.
- Gray plagioclase may show faint parallel lines.
Structure: Coarse-grained (phaneritic)
Full Gabbro guide →More comparisons
Could you tell them apart from a blurry close-up?
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