igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock composed chiefly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, here brown from weathering, with dull to slightly glassy luster and no visible large crystals. Hardness about 6–7 Mohs; dense, massive, and lacking prominent cleavage; specific gravity roughly 2.8–3.0.
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Physical properties
Typically dark gray to black, here brown from weathering, with dull to slightly glassy luster and no visible large crystals. Hardness about 6–7 Mohs; dense, massive, and lacking prominent cleavage; specific gravity roughly 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The rounded shape indicates transport and abrasion in a river, beach, or glacial environment; its exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces may be used decoratively or collected. This specimen appears common rather than gem-grade.
Geological facts
The rusty brown coating likely reflects oxidation of iron-bearing minerals, while black patches may be fresher basalt or weathered pyroxene-rich areas. Vesicles are not obvious, so it is probably relatively dense basalt rather than scoria.
Field identification & locations
Identify by its high density, dark fine-grained texture, magnetic response in some specimens, and resistance to scratching by a steel blade. Common worldwide near volcanic regions and river or coastal gravel deposits.
