igneous

Diabase (Dolerite)

Diabase / Dolerite

Hardness: 6-7 (Mohs). Color: Dark gray to black, often with greenish hues. Luster: Sub-vitreous to dull. Crystal structure: Fine to medium-grained phaneritic (ophitic texture). Cleavage: Two directions at nearly 90 degrees (pyroxene). Specific gravity: 2.7-3.3.

Hardness: 6-7 (Mohs)
Color: Dark gray to black, often with greenish hues
Luster: Sub-vitreous to dull

Identified June 3, 2026More igneous →

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Physical properties

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 cooled more slowly, allowing for visible crystals. Commonly associated with continental rifting zones.

Uses & applications

Used primarily as crushed stone for road construction, railroad ballast, and concrete aggregate. Select varieties are used as ornamental stone and cemetery monuments (‘black granite’).

Geological facts

Many of the bluestones at Stonehenge are made of spotted dolerite. It is significantly tougher than true granite due to its interlocking crystal structure.

Field identification & locations

Identify by its dark color, high density, and slightly granular texture compared to smooth basalt. Widely found in the Palisades Sill (USA), the Karoo province (South Africa), and across the UK.

Other igneous specimens

See all

Granite

Igneous

Granite

Igneous

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dalmatian Jasper

Aplite with Arfvedsonite inclusions

igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous