Diabase (also known as Dolerite)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark gray to black, often with greenish hues or white speckling from feldspar. Luster: Sub-vitreous to dull. Crystal structure: Fine to medium-grained ophitic texture. 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 has a coarser grain size due to slower cooling underground.

Uses & Applications

Commonly used as crushed stone for road construction, railroad ballast, and building foundations. High-quality specimens are sometimes used as 'black granite' in monuments and countertops.

Geological Facts

Diabase is famous for its role in the construction of Stonehenge; the 'bluestones' used in the inner circle are a type of spotted dolerite. It is also the primary rock type of the Palisades sill along the Hudson River.

Field Identification & Locations

Identify it by its heavy weight, dark color, and interlocking grain texture. It won't scratch with a steel nail. Commonly found in areas of rifting or volcanic activity, such as the Appalachian Mountains or the Karoo Basin.

Identified on: 5/8/2026

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