Diabase (also known as Dolerite)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: dark grey to black, often with greenish hues; Luster: vitreous to dull; Crystal structure: ophitic (lath-shaped crystals); Specific gravity: 2.7-3.3.

Formation & Geological History

Formed from the shallow cooling of volcanic magma, typically in dikes or sills. It is chemically similar to basalt but has a coarser grain size due to slower cooling beneath the surface.

Uses & Applications

Widely used as crushed stone for road construction, railroad ballast, and building foundations; some varieties are used as ornamental building stone ('black granite') and gravestones.

Geological Facts

Many of the bluestones used to build Stonehenge are diabase. It is often found in the famous 'Palisades' along the Hudson River in New York/New Jersey.

Field Identification & Locations

Identify by its 'salt and pepper' appearance (though darker than granite) and its extreme toughness. It is commonly found in areas of continental rifting or ancient volcanic activity.

Identified on: 4/26/2026

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