Diabase

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark gray, black, or greenish-gray; Luster: Dull to sub-vitreous; Structure: Fine to medium-grained ophitic texture; Cleavage: None (conchoidal fracture); 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 continental rift zones and volcanic provinces.

Uses & Applications

Extensively used as crushed stone for road construction (road base), railroad ballast, building foundations, and as a high-quality concrete aggregate.

Geological Facts

Diabase is the primary material used to build the inner circle of Stonehenge (known as 'bluestones'). It is chemically identical to basalt and gabbro but differs in grain size.

Field Identification & Locations

Identify in the field by its high density, dark color, and interlocking needle-like crystals (hand lens required). Common in the Palisades Sill of NY/NJ and various regions of the UK and India.

Identified on: 4/30/2026

Mode: Standard