igneous
Gabbro
Gabbro (Coarse-grained mafic intrusive igneous rock)
Hardness: 6-7 (Mohs); Color: Dark gray to black, often with greenish hues; Luster: Dull to sub-vitreous; Crystal Structure: Phaneritic (visible crystals); Cleavage: Prominent in pyroxene/feldspar components; Specific Gravity: 2.7-3.3.
- Hardness
- 6-7 (Mohs)
- Color
- Dark gray to black, often with greenish hues
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray to black, often with greenish hues; Luster: Dull to sub-vitreous; Crystal Structure: Phaneritic (visible crystals); Cleavage: Prominent in pyroxene/feldspar components; Specific Gravity: 2.7-3.3.
Formation & geological history
Formed from the slow cooling of magnesium-rich and iron-rich magma inside the Earth's crust (intrusive). Commonly found in the deep oceanic crust and in large continental volcanic provinces.
Uses & applications
Used frequently as dimension stone (crushed stone) for construction, railroad ballast, and polished as 'black granite' for kitchen countertops and floor tiles.
Geological facts
Gabbro is the chemical equivalent of basalt, but while basalt forms on the surface and is fine-grained, gabbro cools slowly underground to form large crystals. It makes up a significant portion of the Earth's oceanic crust.
Field identification & locations
Look for a dark-colored, coarse-grained rock where the crystals are roughly equal in size. It lacks the quartz typically found in granite. Found in many mountainous regions and ancient oceanic rift sites.
