igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark brown to gray-black, dense, fine-grained, and mostly dull to subtly waxy from weathering; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape indicates prolonged water or sediment abrasion.
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Physical properties
Dark brown to gray-black, dense, fine-grained, and mostly dull to subtly waxy from weathering; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. Its smooth shape indicates prolonged water or sediment abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The specimen’s exact age and source cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are sold as landscape or decorative specimens. This rounded pebble has little jewelry value.
Geological facts
The brown surface likely reflects oxidation of iron-bearing minerals. A magnet may show weak attraction if the basalt contains magnetite.
Field identification & locations
Common along volcanic shorelines, rivers draining volcanic terrain, and glacial deposits. Confirm with a fresh break: basalt should be compact, fine-grained, dark, and heavier than many ordinary sedimentary pebbles; avoid confusing it with slag or dark chert.
