igneous
Weathered basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, locally tan from weathering; dull to faintly subvitreous, compact, and rounded. Typical hardness is about 6 on Mohs scale; crystals are microscopic, with no visible cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray to black, locally tan from weathering; dull to faintly subvitreous, compact, and rounded. Typical hardness is about 6 on Mohs scale; crystals are microscopic, with no visible cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, likely during the Cenozoic or older depending on locality; its smooth shape indicates stream, beach, or glacial abrasion and weathering.
Uses & applications
Common basalt is used as crushed aggregate, road material, riprap, paving stone, and occasionally decorative stone; this small pebble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas. The tan coating likely represents oxidation, clay alteration, or surface staining.
Field identification & locations
Identify it by its heavy, fine-grained black appearance, lack of obvious individual crystals, and inability to scratch it with a steel knife easily. Common worldwide, especially volcanic regions, river gravels, beaches, and oceanic islands.
