igneous
Basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dull to faintly vitreous, with hardness about 5–6 Mohs and specific gravity around 2.8–3.0. The rounded, smooth shape indicates water or abrasion weathering; no obvious crystals are visible.
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Physical properties
Typically dark gray to black, fine-grained, dull to faintly vitreous, with hardness about 5–6 Mohs and specific gravity around 2.8–3.0. The rounded, smooth shape indicates water or abrasion weathering; no obvious crystals are visible.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. Exact age cannot be determined from the photograph.
Uses & applications
Used widely as crushed aggregate, road metal, railway ballast, riprap, building stone, and occasionally decorative cobbles or carvings; ordinary pebbles have minimal jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, pillow lavas, and volcanic plateaus.
Field identification & locations
Identify it by its dark color, high density, fine texture, and lack of visible quartz; a streak, magnet test, and fresh broken surface help distinguish it from slag or dark sedimentary rocks. Common worldwide, especially near volcanic terrains and streambeds.
