igneous
Gabbro (River Cobble)
Phaneritic Mafic Igneous Rock
Hardness: 6-7 Mohs; Color: Dark grey to black with visible flecks of lighter plagioclase; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained) crystalline; Specific Gravity: 2.7-3.3
- Hardness
- 6-7 Mohs
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black with visible flecks of lighter plagioclase; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained) crystalline; Specific Gravity: 2.7-3.3
Formation & geological history
Formed from the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (intrusive). This specific specimen has been smoothed and rounded by fluvial (river) erosion.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally polished as 'black granite' for countertops and grave markers.
Geological facts
Gabbro is the chemical equivalent of basalt, but while basalt forms on the surface, gabbro forms deep underground. It makes up a large portion of the Earth's oceanic crust.
Field identification & locations
Identified by its dark color and coarse, interlocking crystals visible to the naked eye. In the field, look for 'salt and pepper' appearance where the 'salt' is dark grey plagioclase and the 'pepper' is pyroxene or olivine. Common in riverbeds near mountainous or volcanic origins.
