igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to brown with rusty weathering, dull to weakly lustered, very fine-grained and massive, with no obvious cleavage. The rounded shape and pitted surface indicate stream or beach abrasion and weathering.
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Physical properties
Hardness about 6–7 Mohs; dark gray to brown with rusty weathering, dull to weakly lustered, very fine-grained and massive, with no obvious cleavage. The rounded shape and pitted surface indicate stream or beach abrasion and weathering.
Formation & geological history
Basalt forms when low-silica magma cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic fields. Its exact age cannot be determined from the image; weathered cobbles may range from recent to billions of years old.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and paving stone; attractive polished pieces are occasionally collected, though this specimen has limited gem value.
Geological facts
Rust-colored areas commonly result from oxidation of iron-bearing minerals, while small pits may reflect vesicles or weathering. Basalt is typically denser and darker than andesite or granite.
Field identification & locations
Identify it by its heavy weight, dark fine-grained texture, lack of visible large crystals, and inability to scratch easily with a steel knife. Common sources include volcanic regions and basalt-bearing rivers, beaches, and glacial deposits.
