igneous
Tumbled basalt (likely weathered mafic river pebble)
Basalt; predominantly plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark brown to black, finely crystalline, massive, and weakly lustrous from polishing; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.1. The rusty brown color likely reflects surface oxidation and weathering.
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Physical properties
Dark brown to black, finely crystalline, massive, and weakly lustrous from polishing; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.1. The rusty brown color likely reflects surface oxidation and weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. The rounded shape indicates prolonged stream or beach abrasion; its precise age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road stone, riprap, ballast, and building stone; polished pebbles are sold for décor and craftwork. This small specimen has modest collector value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the ocean floor. Magnetic minerals may make some pieces weakly attract a magnet.
Field identification & locations
Identify it by its fine-grained dark interior, heavy feel, lack of obvious large crystals, and possible weak magnetism; a fresh broken surface should be gray-black. Common worldwide, especially near volcanic terrains and derived river gravels.
