igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark brown-black, rounded and waterworn, with dull to subtly glassy patches and scattered lighter mineral grains; typically Mohs 5–6, massive, fine-grained, and lacking visible cleavage. The rusty brown areas likely reflect weathering of iron-bearing minerals.
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Physical properties
Dark brown-black, rounded and waterworn, with dull to subtly glassy patches and scattered lighter mineral grains; typically Mohs 5–6, massive, fine-grained, and lacking visible cleavage. The rusty brown areas likely reflect weathering of iron-bearing minerals.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its rounded shape indicates later transport by water or glacial action. Exact age and locality cannot be determined from the photograph.
Uses & applications
Used as aggregate, roadstone, riprap, and landscaping material; attractive polished pieces may be sold as common decorative stones. This specimen has little commercial value unless locality or unusual texture is documented.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Its dark color and fine grain result from rapid cooling of magnesium- and iron-rich magma.
Field identification & locations
Identify by its dense dark color, fine texture, irregular pale plagioclase grains, and lack of obvious layering or sedimentary cement; a streak, magnetism test, and fresh fracture would help confirm it. Common in volcanic regions, river gravels, beaches, and glacial deposits.
