igneous
Dark basalt cobble
Basalt, predominantly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, generally dull to faintly subvitreous; hardness about 6 on Mohs scale, no obvious cleavage, and typical specific gravity around 2.8–3.0. The rounded shape indicates weathering or stream transport; surface soil obscures diagnostic texture.
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Physical properties
Fine-grained, dark gray to black, generally dull to faintly subvitreous; hardness about 6 on Mohs scale, no obvious cleavage, and typical specific gravity around 2.8–3.0. The rounded shape indicates weathering or stream transport; surface soil obscures diagnostic texture.
Formation & geological history
Extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic episodes. Exact age and locality cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and building or landscaping stone; dense attractive pieces may be collected, though this specimen appears common and unpolished.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of oceanic crust. It may contain vesicles, tiny crystals, or magnetic iron minerals, though none are clearly visible here.
Field identification & locations
Identify it by its uniformly dark color, fine grain, heavy feel, and lack of visible large crystals; a steel file should scratch it only with difficulty, while vinegar produces no reaction. Common worldwide, especially volcanic regions.
