Diabase

Physical Properties

Hardness: 6-7 (Mohs); Color: Dark grey to black, often with greenish hues; Luster: Sub-vitreous to dull; Crystal structure: Fine-to-medium grained phaneritic (ophitic texture); Specific gravity: 2.7–3.3.

Formation & Geological History

Formed by the relatively rapid cooling of basaltic magma in shallow intrusive bodies like sills, dikes, or lopoliths. It is intermediate in cooling rate between basalt and gabbro.

Uses & Applications

Used extensively as crushed stone for road construction, railroad ballast, and high-strength concrete. Higher quality slabs are used as 'black granite' for countertops and headstones.

Geological Facts

Diabase is the primary stone used in the construction of the inner circle of Stonehenge (specifically the 'bluestones'). It is known for its extreme toughness due to its interlocking crystal structure.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and salt-and-pepper appearance under a hand lens highlighting plagioclase feldspar and pyroxene. Common in tectonic rift zones and basaltic provinces.

Identified on: 4/26/2026

Mode: Standard