igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectDark gray, massive, dull to faintly waxy, fine-grained, and likely vesicular or weather-pitted; hardness about 5–6, with no obvious cleavage and specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray, massive, dull to faintly waxy, fine-grained, and likely vesicular or weather-pitted; hardness about 5–6, with no obvious cleavage and specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic flows, or flood-basalt provinces. The specimen is likely Cenozoic or older, but its age cannot be determined visually.
Uses & applications
Used as crushed aggregate, roadstone, riprap, railway ballast, and occasionally building stone; attractive dense cobbles may be collected or used decoratively, but this example has little gem value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals, while weathering commonly produces brown, reddish, or gray surface coatings.
Field identification & locations
Identify by its dark gray color, fine texture, great density, and inability to scratch with a steel knife easily; a streak test is usually gray, and weak magnetism may occur. Common worldwide, especially volcanic regions, river gravels, and beaches.
