igneous
Dark igneous rock fragment, likely basalt
Basalt; commonly composed of plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectDark gray to black, fine- to medium-grained, generally dull to weakly vitreous; hardness about 5–6, no obvious cleavage as a rock, and typically dense (specific gravity ~2.8–3.0). The image is low-resolution, so exact identification is uncertain.
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Physical properties
Dark gray to black, fine- to medium-grained, generally dull to weakly vitreous; hardness about 5–6, no obvious cleavage as a rock, and typically dense (specific gravity ~2.8–3.0). The image is low-resolution, so exact identification is uncertain.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, producing small crystals; it commonly formed from the Precambrian to Recent, including oceanic crust and volcanic flows.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, paving stone, and occasionally decorative stone. Unusual textures may have modest collecting interest.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Vesicles, tiny crystals, or magnetic attraction from magnetite can help distinguish it from similar dark rocks.
Field identification & locations
Look for a uniformly dark, heavy rock with fine interlocking grains; a streak test is usually gray, and dilute acid causes no reaction. Common localities include volcanic regions, oceanic islands, mid-ocean ridges, and flood-basalt provinces.
