igneous
Gabbro (Water-Worn Pebble)
Gabbro (Plagioclase Feldspar and Pyroxene)
Hardness: 6-7 (Mohs scale); Color: Gray to dark green-black with spotted/mottled texture; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray to dark green-black with spotted/mottled texture
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray to dark green-black with spotted/mottled texture; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3
Formation & geological history
Formed from the slow cooling of magnesium-rich and iron-rich magma inside the Earth's crust (intrusive). Commonly found in oceanic crust and mid-ocean ridges. This specific specimen has been shaped and smoothed by river or wave action.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally polished as 'black granite' for countertops or decorative stone.
Geological facts
Gabbro is the intrusive equivalent of basalt; it has the same chemical composition but larger crystals because it cooled slowly underground rather than on the surface.
Field identification & locations
Identify by its dark color and 'salt and pepper' or mottled appearance where the dark minerals (pyroxene) contrast with the light gray minerals (plagioclase). Commonly found on beaches or in glacial till.
