Gabbro
Gabbro is a coarse-grained, dark-colored intrusive igneous rock composed mainly of calcium-rich plagioclase feldspar and pyroxene, with the plutonic equivalent of basalt. It forms deep underground where mafic magma cools slowly enough for its mineral grains to grow large and interlock visibly. Its dense, speckled black-and-white texture makes it a popular dimension stone sold commercially as black granite.
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Gabbro properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 5–6.5 |
| Luster | Dull to vitreous (in crystals) |
| Fracture | Uneven, granular |
| Specific gravity | 2.9–3.1 |
| Transparency | Opaque |
| Common colors | Dark gray, Black, Greenish black, Speckled black and white |
| Rarity | common |
Geology & background
Gabbro crystallizes in large magma chambers at depth, typically in oceanic crust, layered mafic intrusions, and the roots of volcanic systems, where slow cooling allows plagioclase and pyroxene crystals to grow to visible size rather than forming the fine-grained texture of basalt at the surface. It makes up much of the lower oceanic crust, and slices of ancient ocean floor thrust onto land, called ophiolites, expose thick gabbro sequences that geologists use to study seafloor spreading. Notable exposures occur in the Bushveld Complex of South Africa, the Stillwater Complex of Montana, and the Isle of Skye in Scotland.
Rockhounds should note that gabbro's mineral proportions can vary considerably, grading toward diorite as plagioclase becomes more sodium-rich, or toward peridotite as pyroxene and olivine dominate over feldspar.
How to identify Gabbro
Gabbro shows a coarse, phaneritic texture with visible interlocking crystals of dark green to black pyroxene set against light to medium gray plagioclase feldspar, giving the rock an overall dark, speckled appearance. Its luster varies mineral by mineral, vitreous on feldspar cleavage faces and duller on pyroxene, and its streak is generally white or gray since testing the whole rock produces a mix of its component minerals. Hardness runs around 6, and gabbro is commonly confused with basalt, which shares its composition but is fine-grained, or with diorite, which is lighter overall because it contains more feldspar and less pyroxene.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Gabbro is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Gabbro worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- Gabbro is the coarse-grained, slowly cooled equivalent of basalt, sharing the same chemistry but a very different texture.
- Thick sequences of gabbro form much of the lower oceanic crust beneath the world's ocean basins.
- Slabs of gabbro are widely sold in the building trade under the misleading commercial name black granite.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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