igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with minor olivine or magnetite
AI-generated identification · may be incorrectTypically Mohs 6, dark gray to black, dull to weakly subvitreous, and very fine-grained; individual crystals are generally invisible. The specimen is water-rounded, dense, and shows a natural fracture or cooling crack rather than cleavage.
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Physical properties
Typically Mohs 6, dark gray to black, dull to weakly subvitreous, and very fine-grained; individual crystals are generally invisible. The specimen is water-rounded, dense, and shows a natural fracture or cooling crack rather than cleavage.
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. This cobble was likely transported and rounded by a river, beach, or glacial stream; its exact age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are used decoratively. This ordinary cobble has little commercial jewelry value.
Geological facts
Its dark color and fine texture distinguish it from coarse-grained gabbro. A streak test is usually gray, and magnetism may be weak if magnetite is abundant.
Field identification & locations
Common worldwide, especially in volcanic and oceanic regions; notable sources include Iceland, Hawaii, the Columbia River Basalt area, and India’s Deccan Traps. Confirm with a hand lens, density, and absence of visible quartz or large crystals.
