igneous
Weathered basalt cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to brown, fine-grained and typically dull to faintly silky; hardness about 5–6 Mohs, no visible cleavage, massive texture, and specific gravity roughly 2.8–3.0. The orange-brown surface is likely iron-oxide weathering or staining; its rounded form indicates abrasion.
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Physical properties
Dark gray to brown, fine-grained and typically dull to faintly silky; hardness about 5–6 Mohs, no visible cleavage, massive texture, and specific gravity roughly 2.8–3.0. The orange-brown surface is likely iron-oxide weathering or staining; its rounded form indicates abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The specimen’s exact age cannot be determined from the photograph; weathering occurred after exposure and transport.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and building stone; dense, attractive pieces may be used decoratively. This ordinary cobble has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its fine grain reflects rapid cooling, preventing large crystals from forming.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance and resistance to scratching; a steel file usually scratches weakly or not at all. Common in volcanic regions, river gravels, beaches, and glacial deposits.
