igneous
Rounded river pebble, likely basalt or fine-grained volcanic rock
Probable basalt (plagioclase–pyroxene ± olivine), though exact identification requires testing
AI-generated identification · may be incorrectDark brown-gray, fine-grained, dull to slightly earthy luster, and smoothly water-worn; likely Mohs hardness 5–6, with no visible cleavage or crystals. Its dense, compact appearance suggests a specific gravity around 2.8–3.1.
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Physical properties
Dark brown-gray, fine-grained, dull to slightly earthy luster, and smoothly water-worn; likely Mohs hardness 5–6, with no visible cleavage or crystals. Its dense, compact appearance suggests a specific gravity around 2.8–3.1.
Formation & geological history
Probably formed when mafic lava cooled rapidly at or near Earth’s surface, then was transported and rounded by stream or wave action. Basalt ranges from Precambrian to modern in age; the specimen’s age and source cannot be determined from the image.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and occasionally building stone; this pebble has mainly decorative or educational value.
Geological facts
Basalt commonly contains microscopic plagioclase and pyroxene crystals, giving it a uniform appearance even when technically crystalline. A weak attraction to a strong magnet may occur if magnetite is present.
Field identification & locations
Look for a dense, fine texture, lack of obvious layering, and possible tiny dark crystals; a streak, hardness, magnetism, and acid test would improve confidence. Similar brown pebbles may instead be andesite, chert, or weathered sedimentary rock.
