Diabase Identification Guide

Identify diabase (dolerite), a dark medium-grained intrusive igneous rock, by its texture, mineralogy, hardness, and basalt/gabbro look-alikes.

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What Diabase Looks Like

Diabase (also called dolerite) is a dark, medium-grained intrusive igneous rock with the same composition as basalt and gabbro. It is dominated by plagioclase feldspar and pyroxene (augite), giving a dark gray to greenish-black, salt-and-pepper appearance. Its hallmark is an ophitic texture: lath-shaped plagioclase crystals enclosed within larger pyroxene grains, often visible with a loupe.

Key visual cues:

Color: dark gray, greenish-black, to black.
Grain size: medium (crystals visible to the eye/loupe), between fine basalt and coarse gabbro.
Texture: ophitic/subophitic; interlocking, no layering.
Luster: dull to subvitreous on fresh surfaces.

Step-by-Step Field-ID Checklist

Assess grain size. Crystals are visible but not coarse — finer than gabbro, coarser than basalt.
Look for the salt-and-pepper mix of light plagioclase laths and dark pyroxene.
Check for ophitic texture with a loupe — feldspar laths set in pyroxene.
Test hardness. The rock as a whole scratches glass (silicate minerals, Mohs ~5.5–6.5).
Heft it. It feels dense and heavy for its size.

Diagnostic Tests

Mohs hardness: ~6 overall (pyroxene and feldspar); scratches glass, resists a knife.
Streak: gray to greenish, often weak.
Fracture: tough, breaks across grains (no cleavage as a rock).
Density: ~2.9–3.1 g/cm³ — denser than granite.
Acid: inert (no fizz) unless weathered carbonate is present.
Magnetism: often weakly magnetic from accessory magnetite.

Common Look-Alikes and How to Tell Them Apart

Basalt: same composition but fine-grained (crystals not visible) and often vesicular; diabase is medium-grained with visible crystals.
Gabbro: same composition but coarse-grained; diabase is finer. Grain size is the dividing line.
Andesite/diorite: lighter colored and more intermediate; diabase is darker and richer in pyroxene.
Amphibolite (metamorphic): may look similar but shows foliation/alignment of hornblende; diabase is massive and igneous with ophitic texture.
Black granite (commercial): often actually gabbro/diabase; true granite is lighter with quartz and abundant feldspar.

Where It Is Found

Diabase forms in shallow intrusions — dikes, sills, and sheets — where basaltic magma cooled at moderate depth. Classic occurrences include the Palisades Sill (New Jersey/New York), the Whin Sill (England), the Karoo and Ferrar dolerites, and countless dike swarms worldwide. It is extremely tough and widely quarried as crushed stone and 'trap rock' for road aggregate.

Frequently asked questions

How can you tell if it's diabase?

Look for a dark, dense, medium-grained igneous rock of plagioclase and pyroxene with ophitic texture (feldspar laths inside pyroxene), Mohs ~6 hardness that scratches glass, and high density (~3 g/cm³). It is massive, not layered or foliated.

What is the difference between diabase and basalt?

They share the same composition, but basalt is fine-grained (crystals invisible) because it cooled fast at the surface, while diabase cooled more slowly underground and has visible medium-sized crystals.

Is diabase the same as dolerite?

Yes. Diabase and dolerite are two names for the same rock; 'dolerite' is the preferred term in British usage and 'diabase' in North America.

Diabase vs gabbro — how are they different?

Both have the same minerals, but gabbro is coarse-grained (cooled deep and slowly) while diabase is medium-grained (cooled at shallower depth). Grain size separates them.

Diabase identified by the community

Recent Diabase specimens identified with Rock Identifier.