# How to identify gabbro

Canonical page: https://lithomap.app/resources/rock-identification/gabbro
Last reviewed: September 17, 2026
Page purpose: Original LithoMap rock identification reference for field observations, search systems, and language models.
Classification: Mafic intrusive igneous rock

## Summary

Gabbro is a dark, coarse-grained intrusive igneous rock made chiefly of calcium-rich plagioclase and pyroxene, commonly with olivine or iron-titanium oxides.

## Quick identification clues

- Dark rock with crystals visible to the unaided eye
- Interlocking plagioclase and dark mafic minerals
- Coarse-grained counterpart of basalt
- Dense and generally massive rather than layered or vesicular

## Composition and classification

Typical gabbro contains calcium-rich plagioclase plus pyroxene, with variable olivine, amphibole, and oxide minerals. Mineral proportions define related names such as norite, troctolite, and anorthosite, so “gabbroic” may be the most honest field description.

## Appearance

Fresh gabbro is commonly black, dark green, or charcoal gray with paler plagioclase grains. Grain size ranges from a few millimeters to much larger crystals.

Alteration can replace mafic minerals with green amphibole, chlorite, or serpentine and can cloud plagioclase, making old gabbro look mottled or greenstone-like.

## Identification tests

- Grain size: Visible, interlocking crystals. Distinguishes gabbro from fine-grained basalt.
- Color index: A substantial proportion of dark mafic minerals. Usually darker than diorite, although plagioclase-rich varieties overlap.
- Magnetism: May be weakly to noticeably magnetic. Iron oxides can support the identification, but magnetism is variable and not diagnostic alone.

## Common lookalikes

- Basalt: Has crystals too small to identify easily and may contain vesicles; gabbro cooled slowly and is coarse grained.
- Diorite: Usually has a more even salt-and-pepper balance and a less mafic composition.
- Amphibolite: Metamorphic amphibolite commonly shows mineral alignment or foliation and is dominated by amphibole plus plagioclase.

## Direct side-by-side comparisons

- Basalt vs. gabbro — https://lithomap.app/resources/rock-identification/compare/basalt-vs-gabbro
- Amphibolite vs. gabbro — https://lithomap.app/resources/rock-identification/compare/amphibolite-vs-gabbro

## Most relevant related guides

- Amphibolite — https://lithomap.app/resources/rock-identification/amphibolite
- Basalt — https://lithomap.app/resources/rock-identification/basalt
- Diorite — https://lithomap.app/resources/rock-identification/diorite
- Andesite — https://lithomap.app/resources/rock-identification/andesite

## Formation

Gabbro crystallizes slowly from mafic magma below the surface. It forms in layered intrusions, plutons, dikes, and the lower part of oceanic crust.

Crystal settling and repeated magma input can produce igneous layering, but not every striped dark intrusive rock is gabbro.

## Occurrence and geologic setting

Gabbro occurs in oceanic crust exposed on land, mafic intrusive complexes, layered intrusions, and deeply eroded volcanic systems. Glaciers and rivers can move durable blocks far from bedrock.

Dark color alone is insufficient: diabase, amphibolite, basalt, pyroxenite, and altered mafic rocks can be similar.

## Uses and significance

Gabbro and commercially related dark stones are used for aggregate, dimension stone, monuments, and polished architectural surfaces.

## Field safety and limitations

- Use grain size and interlocking texture before color.
- Layered intrusions can contain economically important minerals, but a hand specimen does not demonstrate an ore body.
- Do not enter abandoned mines or active quarries.

## Authoritative sources

- Igneous Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/igneous.htm
- How Rocks Are Classified — U.S. National Park Service: https://www.nps.gov/subjects/geology/how-rocks-are-classified.htm
- Rocks and Minerals — U.S. National Park Service: https://www.nps.gov/subjects/rocks/index.htm
- What is the difference between a rock and a mineral? — U.S. Geological Survey: https://www.usgs.gov/faqs/what-difference-between-a-rock-and-a-mineral

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/rocks/gabbro.webp
- Description: Dark-gray medium-grained gabbro with pale plagioclase and dark mafic minerals
- Creator: Mike Davis
- License: CC0 1.0 — https://creativecommons.org/publicdomain/zero/1.0/
- Source record: https://commons.wikimedia.org/wiki/File:Gabbro_(GeoDIL_number_-_134).jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.