igneous
Rounded basalt cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, massive, and dull to faintly silky; hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape and surface texture indicate prolonged water or sediment abrasion.
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Physical properties
Dark gray-green, fine-grained, massive, and dull to faintly silky; hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape and surface texture indicate prolonged water or sediment abrasion.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, generally from the Cenozoic or older volcanic episodes. The pictured cobble was later transported and rounded by a stream, beach, or glacial environment.
Uses & applications
Commonly used as road aggregate, riprap, landscaping stone, and building material; attractive cobbles may be used decoratively. It generally has little collector value.
Geological facts
Basalt is Earth’s most widespread volcanic rock and forms much of the oceanic crust. Fine texture reflects rapid cooling that prevented large crystals from developing.
Field identification & locations
Identify it by its uniformly fine grain, dark mafic color, high density, and lack of obvious layering or large crystals; a streak, magnetism, or acid test may help distinguish it from similar dark stones. Common in volcanic terrains, riverbeds, beaches, and glacial deposits worldwide.
