igneous
Rounded dark pebble, likely basalt or weathered mafic rock
Basalt; predominantly plagioclase feldspar and pyroxene, possibly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to slightly submetallic surface with rounded weathering; typically hardness 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.1. The image is insufficient for definitive identification because the specimen is small and partly shadowed.
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Physical properties
Dark gray, fine-grained, dull to slightly submetallic surface with rounded weathering; typically hardness 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.1. The image is insufficient for definitive identification because the specimen is small and partly shadowed.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly during volcanic eruptions; individual pieces may be geologically young or millions to billions of years old. Its rounded shape likely reflects transport by water or abrasion.
Uses & applications
Used widely as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished pieces are occasionally used decoratively. This small common pebble has negligible commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms seafloor pillow lavas. Dark vesicles, if present, may indicate gas-rich lava; a streak, magnet test, and fresh broken surface would improve identification.
Field identification & locations
Common worldwide, especially in volcanic regions, riverbeds, beaches, and glacial deposits. Estimated value: $0–$5 as an ordinary specimen; perhaps $5–$20 if unusually aesthetic or provenance-labeled.
