igneous
Rounded dark igneous pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine
AI-generated identification · may be incorrectDark gray-green, smooth and rounded from abrasion, with a dull to subtly waxy luster and tiny pale mineral specks. Typically hardness 6–7 Mohs, no visible cleavage, fine-grained interlocking crystals, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray-green, smooth and rounded from abrasion, with a dull to subtly waxy luster and tiny pale mineral specks. Typically hardness 6–7 Mohs, no visible cleavage, fine-grained interlocking crystals, and specific gravity about 2.8–3.0.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly during Cenozoic to Precambrian volcanic activity. The specimen’s rounded shape indicates later transport by water, ice, or wave action.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and occasionally building stone; attractive polished pieces are collected but rarely used as gemstones.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly contains microscopic crystals that require magnification to see clearly.
Field identification & locations
Identify it by its dark color, substantial weight, fine texture, and lack of obvious layers or large crystals; a streak, magnetism, and acid test can help distinguish it from similar rocks. Common in volcanic regions, river gravels, beaches, and glacial deposits.
