igneous
Weathered basalt pebble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, clinopyroxene, and iron-rich oxides
AI-generated identification · may be incorrectDark brown-black, rounded and weathered, with dull to locally slightly glassy luster and small vesicles or pits; typical hardness is about 5–6 Mohs, with no visible cleavage and fine crystalline texture.
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Physical properties
Dark brown-black, rounded and weathered, with dull to locally slightly glassy luster and small vesicles or pits; typical hardness is about 5–6 Mohs, with no visible cleavage and fine crystalline texture.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly in volcanic flows; the specimen was later rounded by stream or beach abrasion and altered by weathering.
Uses & applications
Basalt is widely used as crushed aggregate, road stone, riprap, and paving material; attractive polished pebbles may be used decoratively, though this small specimen has limited commercial value.
Geological facts
The rusty-brown surface reflects oxidation of iron-bearing minerals, while cavities likely represent former gas bubbles. Basalt is the most abundant volcanic rock in oceanic crust and commonly forms oceanic islands and shield volcanoes.
Field identification & locations
Identify it by its dark color, fine grain, high density, hardness, and occasional vesicles; common sources include volcanic regions, river gravels, and beaches. Estimated value: $2–$10 for an ordinary pebble, or up to about $20 if unusually attractive or locality-labeled.
