igneous
Basalt cobble
Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 6, with no visible cleavage and typical specific gravity of 2.8–3.0. The rounded, smooth surface indicates prolonged water or sediment abrasion, possibly with minor weathering or a thin coating.
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Physical properties
Dark gray-black, fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 6, with no visible cleavage and typical specific gravity of 2.8–3.0. The rounded, smooth surface indicates prolonged water or sediment abrasion, possibly with minor weathering or a thin coating.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic events. Its exact age and locality cannot be determined from the photograph.
Uses & applications
Used as aggregate, road ballast, riprap, paving stone, and occasionally as a landscaping or ornamental stone; generally has low collector value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. A strong magnet may attract it weakly if it contains abundant magnetite.
Field identification & locations
Identify it by its uniformly dark color, fine texture, substantial weight, and lack of layering or large crystals; a streak test is usually gray-black, and acid produces no reaction. Common worldwide in volcanic regions, river gravels, and beaches.
