igneous
Gabbro (Water-worn Pebble)
Phaneritic Mafic Igneous Rock
Hardness: 6-7 on Mohs scale. Color: Dark gray to black with speckled white/gray patches. Luster: Dull (weathered) to vitreous (fresh crystals). Crystal structure: Coarse-grained phaneritic. Cleavage: Prismatic (in pyroxene/amphibole components). Specific gravity: 2.7–3.3.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark gray to black with speckled white/gray patches
- Luster
- Dull (weathered) to vitreous (fresh crystals)
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark gray to black with speckled white/gray patches. Luster: Dull (weathered) to vitreous (fresh crystals). Crystal structure: Coarse-grained phaneritic. Cleavage: Prismatic (in pyroxene/amphibole components). 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 has been smoothed by fluvial or glacial transport. Age varies by location but occurs throughout Earth's history.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally polished as 'black granite' for countertops and floor tiles.
Geological facts
Gabbro is the intrusive equivalent of basalt. It is a major constituent of the lower oceanic crust and is much less common on the continental crust than granite.
Field identification & locations
Identified in the field by its dark color, heavy weight (dense), and visible interleaved dark (pyroxene/olivine) and light (plagioclase) crystals. Found in oceanic crust exposures and mountain belts.
