igneous
Rounded dark river pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to subtly vitreous; hardness about 5–6 Mohs, with no visible cleavage and generally high specific gravity (~2.8–3.1). The rounded shape and surface smoothing indicate water or sediment abrasion.
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Physical properties
Typically dark gray to black, fine-grained, and dull to subtly vitreous; hardness about 5–6 Mohs, with no visible cleavage and generally high specific gravity (~2.8–3.1). The rounded shape and surface smoothing indicate water or sediment abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Most examples range from recent to Mesozoic or older, but this individual’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally building stone; ordinary pebbles have little jewelry value.
Geological facts
Its dark color and relatively heavy feel distinguish it from many light-colored quartz-rich rocks, though weathering can produce gray, brown, or reddish surfaces.
Field identification & locations
Common in volcanic regions and streambeds worldwide; inspect a fresh chip for a uniformly fine texture and test hardness against glass. Definitive identification may require a thin section or chemical analysis.
