igneous
Rounded basalt cobble
Basalt; chiefly plagioclase feldspar, clinopyroxene, and iron-magnesium minerals
AI-generated identification · may be incorrectDark gray to brown, fine-grained, massive, and weakly weathered with a dull to slightly waxy luster; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The smooth spherical shape reflects prolonged water abrasion rather than crystal structure.
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Physical properties
Dark gray to brown, fine-grained, massive, and weakly weathered with a dull to slightly waxy luster; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The smooth spherical shape reflects prolonged water abrasion rather than crystal structure.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the specimen was later rounded in a river, beach, or glacial environment.
Uses & applications
Used widely as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished basalt is also used decoratively. This ordinary cobble has little gem or collector value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color results from abundant ferromagnesian minerals and commonly tiny iron-oxide grains.
Field identification & locations
Identify it by its dense, fine-grained dark appearance, lack of visible large crystals, and resistance to scratching; a steel file may scratch it only slightly. Common in volcanic regions, river gravels, beaches, and glacial deposits.
