igneous
Tumbled Gabbro
Gabbro (Mafic intrusive igneous rock)
Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green with lighter mottling; Luster: Sub-vitreous to dull; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3.
- Hardness
- 6-7 Mohs
- Color
- Dark grey, black, or dark green with lighter mottling
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green with lighter mottling; Luster: Sub-vitreous to dull; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3.
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (plutonic). Typically associated with oceanic crust and mid-ocean ridges.
Uses & applications
Used as 'black granite' in construction (countertops, floor tiles), crushed stone for road base, and as tumbled decorative stones in the gemstone trade.
Geological facts
Gabbro is the chemical equivalent of basalt but differs in crystal size because basalt cools rapidly on the surface while gabbro cools slowly underground. Large parts of the Earth's oceanic crust are made of gabbro.
Field identification & locations
Identify in the field by its dark color, heavy weight, and visible interlocking crystals of plagioclase feldspar and pyroxene. Commonly found in ophiolites and deep mountain roots.
