igneous
Black river pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, rounded and water-worn, with a dull to faintly glassy luster; typically hardness 5–6 Mohs, dense, fine-grained, and lacking visible cleavage or crystals at this size.
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Physical properties
Dark gray to black, rounded and water-worn, with a dull to faintly glassy luster; typically hardness 5–6 Mohs, dense, fine-grained, and lacking visible cleavage or crystals at this size.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, generally during the Cenozoic or older volcanic episodes; the rounded shape indicates transport by water or sediment.
Uses & applications
Used as aggregate, roadstone, ballast, riprap, and sometimes decorative landscaping stone; ordinary pebbles have negligible jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms pillow lavas underwater.
Field identification & locations
Identify it by its uniform black color, high density, fine texture, and lack of fizz with dilute hydrochloric acid; possible alternatives include dark chert, slag, or coal, which require closer inspection or testing. Common in volcanic regions, riverbeds, and glacial deposits.
