igneous
Dark igneous rock, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically black to dark gray, dull to slightly vitreous, fine-grained, and massive; hardness about 6 on the Mohs scale, with no obvious cleavage and specific gravity roughly 2.8–3.0. The image is low-resolution, so exact identification is uncertain.
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Physical properties
Typically black to dark gray, dull to slightly vitreous, fine-grained, and massive; hardness about 6 on the Mohs scale, with no obvious cleavage and specific gravity roughly 2.8–3.0. The image is low-resolution, so exact identification is uncertain.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface, commonly during volcanic eruptions; specimens may range from recent to hundreds of millions of years old. The rounded shape appears naturally weathered or possibly tumbled.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, building stone, and occasionally landscaping material; attractive polished pieces are used decoratively.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. Vesicles, tiny crystals, or scattered olivine grains may be visible on a fresh broken surface.
Field identification & locations
Look for a dense, dark, fine-grained texture, lack of layering, and a heavy feel; a streak test is usually gray to black. Common worldwide, especially in volcanic regions and oceanic crust.
