igneous
Rounded river pebble, likely weathered basalt or andesite
Fine-grained mafic volcanic rock; probable basalt (plagioclase + pyroxene ± olivine, altered by weathering)
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dull earthy brown exterior from weathering, with a fine-grained texture and no visible crystals or cleavage. Its smooth, oval form reflects abrasion in flowing water rather than crystal structure.
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Physical properties
Hardness about 5.5–6.5 Mohs; dull earthy brown exterior from weathering, with a fine-grained texture and no visible crystals or cleavage. Its smooth, oval form reflects abrasion in flowing water rather than crystal structure.
Formation & geological history
Likely cooled rapidly as a lava flow during the Cenozoic or older, then weathered and rounded in a stream, beach, or glacial deposit; exact age and identification require a fresh broken surface or laboratory testing.
Uses & applications
Common basalt is used for aggregate, road stone, riprap, and construction; this individual pebble has mainly educational or modest collecting value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust. Brown coatings can conceal a dark gray or black interior.
Field identification & locations
Look for a dense, fine-grained interior, gray-black fresh color, and weak magnetism; a streak, acid test, and hand lens can help distinguish it from sandstone or chert. Common worldwide, especially volcanic regions and river gravels.
