igneous
Dark coarse-grained rock, likely basalt or gabbro
Basalt (fine-grained) or gabbro (coarse-grained), composed mainly of plagioclase feldspar and pyroxene
AI-generated identification · may be incorrectDark gray-brown, massive, and granular with a dull to weakly vitreous luster; likely Mohs hardness 5–6, little visible cleavage as a whole rock, and specific gravity about 2.8–3.1. The image resolution prevents confident distinction between basalt and gabbro.
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Physical properties
Dark gray-brown, massive, and granular with a dull to weakly vitreous luster; likely Mohs hardness 5–6, little visible cleavage as a whole rock, and specific gravity about 2.8–3.1. The image resolution prevents confident distinction between basalt and gabbro.
Formation & geological history
Formed from mafic magma rich in iron and magnesium. Basalt crystallizes rapidly at or near the surface, whereas gabbro crystallizes slowly underground; ages may range from recent volcanic material to ancient Precambrian rocks.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and dimension stone; attractive polished pieces may be used decoratively. Ordinary specimens have minimal commercial value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Gabbro is its coarse-grained intrusive equivalent.
Field identification & locations
Look for a dark, heavy, nonlayered rock with feldspar and pyroxene grains; a hand lens can distinguish coarse gabbro from fine basalt. Common worldwide, especially volcanic regions and oceanic crust; the specimen appears possibly polished or cut.
