igneous
Gabbro
Gabbro
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, or dark greenish (due to pyroxene/olivine); Luster: Phaneritic (coarse-grained visible crystals); Crystal structure: Holocrystalline; Cleavage: Prismatic in two directions at roughly 90 degrees; Specific Gravity: 2.7–3.3
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey, black, or dark greenish (due to pyroxene/olivine)
- Luster
- Phaneritic (coarse-grained visible crystals)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, or dark greenish (due to pyroxene/olivine); Luster: Phaneritic (coarse-grained visible crystals); Crystal structure: Holocrystalline; Cleavage: Prismatic in two directions at roughly 90 degrees; Specific Gravity: 2.7–3.3
Formation & geological history
Formed through the slow cooling of magnesium-rich and iron-rich magma beneath the Earth's surface (plutonic). It is chemically equivalent to basalt but with larger crystals. It is typically found in oceanic crust and large layered intrusions.
Uses & applications
Used widely as crushed stone for road construction and railroad ballast. In its polished form, it is sold as 'black granite' for kitchen countertops, floor tiles, and cemetery monuments.
Geological facts
Much of the Earth's lower oceanic crust is composed of gabbro. It is often found in ophiolites—sections of oceanic crust that have been uplifted onto land through tectonic activity.
Field identification & locations
Identify in the field by its dark color, coarse-grained texture (crystals visible to the naked eye), and high density. Look for it in mountainous areas where ancient oceanic crust has been exposed.
