igneous
Diabase (Dolerite)
Diabase / Dolerite
Hardness: 6-7 (Mohs). Color: Dark gray to black, often with greenish hues. Luster: Sub-vitreous to dull. Crystal structure: Fine to medium-grained phaneritic (ophitic texture). Cleavage: Two directions at nearly 90 degrees (pyroxene). Specific gravity: 2.7-3.3.
- Hardness
- 6-7 (Mohs)
- Color
- Dark gray to black, often with greenish hues
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark gray to black, often with greenish hues. Luster: Sub-vitreous to dull. Crystal structure: Fine to medium-grained phaneritic (ophitic texture). Cleavage: Two directions at nearly 90 degrees (pyroxene). Specific gravity: 2.7-3.3.
Formation & geological history
Formed through the cooling of mafic magma in shallow intrusive bodies like dikes and sills. It is chemically similar to basalt but cooled more slowly, allowing for visible crystals. Commonly associated with continental rifting zones.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and concrete aggregate. Select varieties are used as ornamental stone and cemetery monuments (‘black granite’).
Geological facts
Many of the bluestones at Stonehenge are made of spotted dolerite. It is significantly tougher than true granite due to its interlocking crystal structure.
Field identification & locations
Identify by its dark color, high density, and slightly granular texture compared to smooth basalt. Widely found in the Palisades Sill (USA), the Karoo province (South Africa), and across the UK.
