igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, rounded, dull to faintly satin luster with subtle fine grains and weathering stains; typically Mohs 6, massive, without visible cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray-brown, rounded, dull to faintly satin luster with subtle fine grains and weathering stains; typically Mohs 6, massive, without visible cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
Formed when low-silica lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; this specimen was rounded by stream or shoreline transport and surface weathering.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and paving material; attractive polished pieces may be sold as display stones, but this appears commercially common.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, pillow lavas, and volcanic plateaus.
Field identification & locations
Identify it by its heavy, fine-grained dark body, lack of quartz-like sparkle, and resistance to scratching with a steel blade; common in volcanic regions and river gravels worldwide.
