igneous
Weathered basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectDark gray-brown, rounded and weathered, with a dull to faintly glassy luster and no visible large crystals. Typical hardness is about 5–6 Mohs; generally massive, dense, and lacking cleavage.
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Physical properties
Dark gray-brown, rounded and weathered, with a dull to faintly glassy luster and no visible large crystals. Typical hardness is about 5–6 Mohs; generally massive, dense, and lacking cleavage.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; its rounded shape indicates transport and abrasion in a stream, beach, or glacial setting.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and occasionally dimension stone; ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly contains microscopic iron- and magnesium-rich minerals that produce its dark color.
Field identification & locations
Identify it by its fine grain, high density, dark color, and resistance to scratching; a steel file usually scratches it only weakly or not at all. Common in volcanic regions, riverbeds, beaches, and glacial deposits.
