igneous
Water-worn basalt cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectTypically dark gray to brown, fine-grained, dense, and matte to slightly vitreous when wet; hardness about 5–6 Mohs, no visible cleavage, massive texture, and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate prolonged water abrasion.
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Physical properties
Typically dark gray to brown, fine-grained, dense, and matte to slightly vitreous when wet; hardness about 5–6 Mohs, no visible cleavage, massive texture, and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate prolonged water abrasion.
Formation & geological history
An extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older depending on source. It was later transported and rounded in a river, lake, or coastal environment.
Uses & applications
Used as aggregate, road stone, riprap, ballast, and occasionally landscaping material; attractive polished pieces may be collected but are rarely used as gemstones.
Geological facts
Basalt is the most abundant volcanic rock and forms much of oceanic crust. A wet surface can make it appear unusually glossy and darker than when dry.
Field identification & locations
Look for fine grain, high density, dark color, and lack of layering or large crystals; a streak test is usually gray. Common worldwide, especially volcanic regions and waterways; magnetism may be weakly detectable from iron minerals.
