igneous
Dark basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with minor olivine or magnetite
AI-generated identification · may be incorrectHardness about 6 on the Mohs scale; dark gray to charcoal, matte to faintly silky, dense, and fine-grained with no visible cleavage. The rounded, smooth surface is beach-worn and may show tiny pale feldspar grains or weathering spots.
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Physical properties
Hardness about 6 on the Mohs scale; dark gray to charcoal, matte to faintly silky, dense, and fine-grained with no visible cleavage. The rounded, smooth surface is beach-worn and may show tiny pale feldspar grains or weathering spots.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions or ancient oceanic crust formation. The specimen is likely geologically old but its exact age and source cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are used decoratively. Ordinary beach pebbles have little commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its dark color, high density, fine texture, and lack of layering or obvious large crystals; it should not fizz with dilute acid. Common on volcanic and glacial beaches worldwide.
