igneous
Tumbled basalt pebble
Basalt, a fine-grained mafic volcanic rock
AI-generated identification · may be incorrectTypically 5–6 Mohs; dark gray to black, with dull to slightly glassy luster and microscopic interlocking plagioclase and pyroxene crystals. The pale outer area may be weathered basalt or an adherent coating, and the rounded shape indicates water abrasion or tumbling.
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Physical properties
Typically 5–6 Mohs; dark gray to black, with dull to slightly glassy luster and microscopic interlocking plagioclase and pyroxene crystals. The pale outer area may be weathered basalt or an adherent coating, and the rounded shape indicates water abrasion or tumbling.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in Cenozoic to modern volcanic settings. The specimen is likely naturally water-worn or mechanically tumbled.
Uses & applications
Used as crushed aggregate, road stone, riprap, paving, and decorative landscaping material; attractive pieces are sold as polished or tumbled stones for collecting.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals, though weathering commonly produces brown, gray, or pale surfaces.
Field identification & locations
Identify it by high density, fine texture, dark color, and absence of visible large crystals; a steel nail generally does not scratch fresh basalt. Common sources include volcanic islands, river gravels, and ocean beaches; exact identification is uncertain from the photograph alone.
