igneous
Dark river-worn stone, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectVery dark gray to black, smoothly rounded and weakly lustered, with no visible crystals; typical hardness is about 6 on the Mohs scale, massive texture, and poor or absent cleavage. Exact identification is uncertain from the photograph alone.
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Physical properties
Very dark gray to black, smoothly rounded and weakly lustered, with no visible crystals; typical hardness is about 6 on the Mohs scale, massive texture, and poor or absent cleavage. Exact identification is uncertain from the photograph alone.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. The smooth shape indicates prolonged abrasion in a river, beach, or glacial environment; its precise age cannot be determined visually.
Uses & applications
Used as crushed aggregate, railroad ballast, road base, riprap, and occasionally as architectural stone or a decorative pebble; this specimen has little jewelry use.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its fine texture results from rapid cooling that prevents large crystals from growing.
Field identification & locations
Look for a dense black-to-gray, fine-grained stone that is heavier than similarly sized sedimentary rocks and may show tiny pale plagioclase flecks or vesicles. Common sources include volcanic regions, river gravels, lake shores, and glacial deposits.
