RocksdayDiabase vs Gabbro
How to tell them apart
Basalt's medium-grained cousin, formed in shallow sills and dikes.
Photo: CrankyScorpion at English Wikipedia, 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 coarser.
Side by side
| Property | Diabase | Gabbro |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 6–6.5 |
| Luster | Granular | Granular |
| Colors | Black, Gray, Green | Black, Gray, Green |
| Crystal system / structure | Medium-grained | Coarse-grained (phaneritic) |
| Specific gravity | 2.9–3.1 | 2.9–3.1 |
| Found in | Palisades, New Jersey; Preseli Hills, Wales | Duluth Complex, Minnesota; Cuillin, Isle of Skye |
Properties that are the same for both are grayed out.
How to identify diabase
- Dark gray to black with small visible crystals.
- Heavy and tough.
- Lath-shaped feldspar between dark pyroxene.
Structure: Medium-grained
Full Diabase 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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