igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock; chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark brown to black, dense, fine-grained, and generally dull to weakly vitreous; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The pale specks may be weathered feldspar or small vesicle fillings, though the image is insufficient for certainty.
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Physical properties
Dark brown to black, dense, fine-grained, and generally dull to weakly vitreous; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The pale specks may be weathered feldspar or small vesicle fillings, though the image is insufficient for certainty.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; the rounded shape indicates transport and abrasion in a stream, beach, or sediment deposit.
Uses & applications
Basalt is widely used as crushed aggregate, road metal, railroad ballast, building stone, and occasionally as a decorative material. This small weathered pebble has mainly modest educational or collecting value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Its iron- and magnesium-rich minerals commonly weather to brown or reddish coatings.
Field identification & locations
Identify it by its dark, heavy, fine texture and lack of obvious layering; a steel nail may scratch it only with difficulty. Common sources include volcanic regions, river gravels, and beaches worldwide.
