igneous
Diabase (Trap Rock)
Diabase (also known as Dolerite)
Hardness: 6-7 on Mohs scale; Color: dark grey to black with greenish/brownish weathering; Luster: dull to sub-metallic; Crystal structure: fine to medium-grained phaneritic (interlocking laths of plagioclase); Specific Gravity: 2.7-3.3.
- Hardness
- 6-7 on Mohs scale
- Color
- dark grey to black with greenish/brownish weathering
- Luster
- dull to sub-metallic
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Physical properties
Hardness: 6-7 on Mohs scale; Color: dark grey to black with greenish/brownish weathering; Luster: dull to sub-metallic; Crystal structure: fine to medium-grained phaneritic (interlocking laths of plagioclase); Specific Gravity: 2.7-3.3.
Formation & geological history
Formed by the relatively rapid cooling of mafic magma in shallow intrusive bodies like sills, dikes, and lopoliths. Common in the Palisades Sill (NY/NJ) and Karoo Province.
Uses & applications
Crushed stone for road construction, railroad ballast, and high-durability building stone (sometimes sold as 'Black Granite').
Geological facts
Diabase is what makes up the famous 'Bluestones' of Stonehenge. It has the same chemical composition as basalt but a larger grain size due to slower cooling underground.
Field identification & locations
Identify in the field by its high density and 'salt and pepper' appearance (though much darker than granite). Often found in linear outcrops where magma forced its way through older rock layers.
