Gabbro Identification Guide

How to identify gabbro by its dark, coarse-grained, equigranular texture of plagioclase and pyroxene, with density and mineral tests that separate it from basalt and diorite.

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What Gabbro Looks Like

Gabbro is a dark, coarse-grained intrusive (plutonic) igneous rock — the slow-cooled, deep-formed equivalent of basalt. It is composed chiefly of calcium-rich plagioclase feldspar and pyroxene (augite), with possible olivine, hornblende, and minor magnetite/ilmenite.

Color: dark grey to greenish-black to nearly black; salt-and-pepper where pale plagioclase mixes with dark pyroxene.
Texture: phaneritic — individual mineral grains are visible to the naked eye, interlocking and roughly equal in size (equigranular). No quartz to speak of, and feldspar is dark grey, not pink.
Form: massive, no layering or vesicles; occurs in large intrusions, dikes, and sills.

Step-by-Step Field-ID Checklist

Check grain size: you can see interlocking crystals with the naked eye — coarse, not glassy or fine.
Assess color: overall dark, mafic, salt-and-pepper.
Identify minerals: dull-to-blocky dark grey plagioclase + black/greenish pyroxene (and sometimes glassy green olivine). Little or no quartz.
Heft it: noticeably dense and heavy.
Test magnetism: often weakly magnetic from magnetite content.

Key Diagnostic Tests

Mohs hardness: overall ~6–7 (made of feldspar ~6 and pyroxene ~5.5–6.5); scratches glass.
Density: high, ~2.9–3.1 g/cm3 — heavier than granite, a useful field clue.
Acid: no reaction (silicate, not carbonate).
Magnetism: commonly weakly magnetic.
Mineral test: plagioclase may show fine striations (twinning) on cleavage faces; pyroxene shows cleavage near 90 degrees.

Common Look-Alikes and How to Tell Them Apart

Basalt: same chemistry but fine-grained (extrusive) — you cannot see individual crystals, and it is often vesicular. If grains are invisible to the eye, it is basalt; if coarse, gabbro.
Diorite: also coarse-grained and salt-and-pepper, but diorite is more intermediate — it has more white/grey plagioclase, less mafic mineral, often a more black-and-white (not greenish) look, and may contain a little quartz or hornblende. Gabbro is darker and denser with calcium-rich plagioclase.
Granite: light-colored, contains abundant quartz and often pink K-feldspar; gabbro lacks quartz and is dark. Color and quartz content separate them instantly.
Peridotite/dunite: even more mafic/ultramafic, dominated by green olivine with little or no feldspar; gabbro retains abundant plagioclase.
Norite: a gabbro variant where the pyroxene is orthopyroxene rather than augite; essentially indistinguishable in the field without thin-section work.

The diagnostic combination is coarse visible grains + dark mafic color + plagioclase-plus-pyroxene mineralogy + little/no quartz + high density.

Where Gabbro Is Found

Gabbro forms from slow cooling of basaltic magma at depth. It is a major component of the lower oceanic crust and ophiolites, and forms large layered intrusions such as the Bushveld Complex (South Africa), Stillwater (Montana, USA), Skaergaard (Greenland), and the Duluth Complex (Minnesota). It is also quarried as 'black granite' dimension stone.

Frequently asked questions

How can you tell if a rock is gabbro?

Gabbro is a dark, coarse-grained igneous rock in which you can see interlocking crystals with the naked eye, made mostly of grey plagioclase feldspar and black or greenish pyroxene with little or no quartz. It is dense and heavy, scratches glass, does not fizz in acid, and is often weakly magnetic.

What is the difference between gabbro and basalt?

They have the same chemistry, but gabbro cooled slowly underground and is coarse-grained with visible crystals, while basalt cooled quickly at the surface and is fine-grained with crystals too small to see, often with gas bubbles (vesicles). If you can see the grains, it is gabbro; if not, basalt.

How do you tell gabbro from diorite?

Both are coarse-grained and salt-and-pepper, but gabbro is darker and denser with calcium-rich plagioclase and abundant pyroxene, while diorite is more black-and-white with more pale plagioclase, less mafic mineral, and sometimes a little quartz. Gabbro's overall darker, heavier character is the main clue.

Is gabbro the same as black granite?

In the stone trade, dark gabbro and similar rocks are often sold as 'black granite,' but geologically they are not granite. True granite is light-colored and quartz-rich, while gabbro is dark and quartz-poor.

What does gabbro look like?

It looks like a dark grey to greenish-black, heavy, coarse-grained rock with a salt-and-pepper texture of visible pale feldspar and dark pyroxene crystals, and no layering or gas bubbles.

Gabbro identified by the community

Recent Gabbro specimens identified with Rock Identifier.