igneous
Gabbroic rock, possibly hornblende gabbro
Gabbro; coarse-grained mafic silicate rock dominated by plagioclase and pyroxene, with possible hornblende and magnetite
AI-generated identification · may be incorrectDark gray, coarse-grained, massive, with black mafic minerals and pale gray plagioclase; generally Mohs 6–7, vitreous to dull luster, little or no cleavage as a rock, and specific gravity about 2.9–3.2.
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Physical properties
Dark gray, coarse-grained, massive, with black mafic minerals and pale gray plagioclase; generally Mohs 6–7, vitreous to dull luster, little or no cleavage as a rock, and specific gravity about 2.9–3.2.
Formation & geological history
Forms when basaltic magma cools slowly deep within the crust, allowing visible crystals to grow; specimens may range from Precambrian to modern oceanic-crust age. The image could also represent altered gabbro or a related dioritic rock.
Uses & applications
Used as crushed aggregate, road stone, dimension stone, countertops, and decorative rock; attractive polished pieces have modest collecting value.
Geological facts
Gabbro is the intrusive equivalent of basalt and commonly contains iron- and magnesium-rich minerals that produce the dark patches. Black areas may be hornblende, pyroxene, biotite, or magnetite.
Field identification & locations
Identify by its interlocking coarse crystals, dark color, high density, and lack of layering or vesicles; a streak, magnet test, and fresh broken surface help distinguish it from basalt and metamorphic rocks. Common in continental intrusions and oceanic crust.
