igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with minor olivine or iron oxides
AI-generated identification · may be incorrectDark gray-brown, compact, fine-grained, and dull to subtly granular; typically Mohs hardness 6, with no visible cleavage and a specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
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Physical properties
Dark gray-brown, compact, fine-grained, and dull to subtly granular; typically Mohs hardness 6, with no visible cleavage and a specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. This specimen was likely rounded in a river, beach, or glacial environment.
Uses & applications
Used as crushed aggregate, road stone, riprap, landscaping material, and occasionally in polished stone products; this small pebble has modest decorative or educational value.
Geological facts
Basalt is Earth’s most common volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals and iron-rich volcanic glass or oxides.
Field identification & locations
Look for a uniformly dark, fine-grained texture, high density, and lack of obvious layers or large crystals; a streak test is usually gray. Common worldwide near volcanic terrains and in transported gravels.
