Dolerite (Diabase)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to black, often with a greenish tint; Luster: Dull to sub-vitreous; Structure: Fine-to-medium grained phaneritic/ophitic texture; Cleavage: None (sub-conchoidal fracture); Specific Gravity: 2.7-3.3

Formation & Geological History

Formed by the shallow intrusive cooling of mafic magma, often in dikes or sills. It is chemically equivalent to volcanic basalt and plutonic gabbro. These formations can range from Pre-Cambrian to Mesozoic ages depending on the location of the intrusion.

Uses & Applications

Extensively used as crushed stone (blue metal) for road base, railroad ballast, and concrete aggregate. High-quality slabs are used for laboratory bench tops, dimension stone in building facades, and occasionally as headstones.

Geological Facts

Large-scale dolerite deposits are part of massive geological features like the Karoo Large Igneous Province or the Palisades Sill in New York. The famous inner circle of Stonehenge is made of 'bluestone,' which is a variety of dolerite.

Field Identification & Locations

Identify in the field by its dark color, high density, and grain size that is visible to the naked eye but smaller than granite. Look for it in vertical dikes cutting through other rock layers. Commonly found in rift zones or areas of ancient tectonic activity.

Identified on: 4/18/2026

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