igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, dull to subtly granular, with a weathered rind and smoothly rounded water-worn shape. Typical hardness is about 6 Mohs; massive, fine-grained, with no visible cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-brown, dull to subtly granular, with a weathered rind and smoothly rounded water-worn shape. Typical hardness is about 6 Mohs; massive, fine-grained, with no visible cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly during volcanic eruptions; its age cannot be determined from the photograph. The rounded form indicates prolonged transport in a river, beach, or glacial environment.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, paving stone, and occasionally architectural stone. Ordinary cobbles have little jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, fine texture, heavy feel, lack of obvious layers, and resistance to scratching by a steel knife; a streak, fresh broken surface, and magnet test can help distinguish it from other dark rocks. Common worldwide, especially near volcanic terrains and river beds.
