igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, typically plagioclase + clinopyroxene ± olivine and magnetite
AI-generated identification · may be incorrectHardness about 6–6.5 Mohs; dark brown to black, weathered/rust-stained surface, dull to faintly silky luster, very fine-grained massive texture, no visible cleavage. The smooth shape indicates water tumbling or abrasion.
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Physical properties
Hardness about 6–6.5 Mohs; dark brown to black, weathered/rust-stained surface, dull to faintly silky luster, very fine-grained massive texture, no visible cleavage. The smooth shape indicates water tumbling or abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact geological age cannot be determined from the photograph; weathering produced the brown coating.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, and dimension stone; attractive polished pieces are collected or used decoratively. This small common pebble has minimal commercial value.
Geological facts
Magnetite may cause weak magnetism, and iron oxidation commonly produces the brown coloration. A streak, density, and thin-section examination would be needed for confident identification.
Field identification & locations
Look for a dense, dark, fine-grained rock with tiny scattered crystals, irregular fracture, and no layering or strong foliation; a steel file generally scratches it only with difficulty. Common in volcanic regions and stream or beach gravels worldwide.
