Gabbro
Mafic intrusive igneous rock (plagioclase + pyroxene)
A coarse-grained, dark mafic intrusive rock that is the plutonic equivalent of basalt, rich in plagioclase and pyroxene.
- Mohs hardness
- 6-7
- Color
- dark grey to greenish-black, often mottled
- Type
- igneous
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Overview
Gabbro is a coarse-grained, dark-colored intrusive igneous rock and the plutonic equivalent of basalt. It is composed mainly of calcium-rich plagioclase feldspar and pyroxene, often with olivine and lesser amounts of other minerals, giving it a dark grey to greenish-black, mottled appearance.
Because it cools slowly deep underground, its mineral crystals are large enough to see with the naked eye. Gabbro is a major component of the lower oceanic crust.
A banded, mottled variety from Madagascar is sold in the lapidary trade as indigo gabbro or "mystic merlinite."
Formation & geology
Gabbro forms when mafic (iron- and magnesium-rich) magma cools and crystallizes slowly deep within the Earth's crust. The slow cooling allows large interlocking crystals of plagioclase and pyroxene to grow, producing its coarse texture.
It is the dominant rock of the lower oceanic crust, forming at mid-ocean ridges, and also occurs in large layered intrusions and plutons on continents. When the same magma reaches the surface and cools quickly it forms basalt instead. Major occurrences include the Bushveld Complex, Stillwater Complex, and ophiolite sequences.
How to identify it
Identify gabbro by its coarse, visibly crystalline texture and dark grey to greenish-black color, with interlocking grains of dark pyroxene and lighter plagioclase. Overall hardness is around 6-7. It is dense and heavy.
It resembles diorite but is darker and more mafic (diorite has more light-colored feldspar and a salt-and-pepper look). It differs from basalt in being coarse-grained rather than fine.
Granite is much lighter and quartz-rich; gabbro contains little to no quartz. The dark, coarse, quartz-poor texture is diagnostic.
Uses & significance
Gabbro is widely used as crushed stone for road base, railroad ballast, and concrete aggregate. Polished, it is sold commercially as "black granite" for countertops, tiles, monuments, and tombstones.
Decorative Madagascar gabbro (indigo gabbro) is cut into cabochons, spheres, and tumbled stones for jewelry and collections.
Metaphysically indigo gabbro is associated with intuition and grounding, though such properties are not scientifically supported. Its durability makes it a valued dimension and building stone.
Frequently asked questions
Is gabbro the same as basalt?
They share composition, but gabbro is coarse-grained and cooled slowly underground, while basalt is fine-grained and cooled quickly at the surface.
Why is gabbro sold as black granite?
Commercially, dark coarse igneous rocks like gabbro are marketed as black granite for countertops and monuments, though geologically they differ.
What is indigo gabbro?
Indigo gabbro, or mystic merlinite, is a mottled blue-black gabbro from Madagascar used as a decorative lapidary stone.
Where is gabbro found?
It forms the lower oceanic crust and occurs in large layered intrusions like the Bushveld and Stillwater complexes.
Gabbro guides
In-depth guides for identifying, valuing, and understanding Gabbro.
Gabbro identified by the community
Real specimens identified with Rock Identifier.
