igneous
Gabbro (River Cobble)
Gabbro (mafic intrusive rock)
Hardness: 6-7 Mohs; Color: Dark grey, green, or black with brownish weathering; Luster: Dull/Sub-metallic (weathered surface); Structure: Phaneritic (coarse-grained crystals); Specific Gravity: 2.7-3.3
- Hardness
- 6-7 Mohs
- Color
- Dark grey, green, or black with brownish weathering
- Luster
- Dull/Sub-metallic (weathered surface)
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey, green, or black with brownish weathering; Luster: Dull/Sub-metallic (weathered surface); Structure: Phaneritic (coarse-grained crystals); 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). This specific specimen is a river cobble, rounded by water erosion.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as high-quality countertop material often sold as 'Black Granite'.
Geological facts
Gabbro is the chemical equivalent of basalt but cooled slowly underground. It makes up a large portion of the Earth's deep oceanic crust.
Field identification & locations
Identify by its dark, heavy nature and visible interlocking crystals of dark minerals like pyroxene. Found commonly in mountainous regions or riverbeds downstream from igneous outcrops.
