igneous
Dark gray rounded river pebble
Likely basalt or fine-grained volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectFine-grained, medium-dark gray, dull to slightly silky luster, rounded and smooth from abrasion; likely Mohs hardness 5–6, with no visible cleavage. The uniform texture may also fit a dark sedimentary or metamorphic pebble.
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Physical properties
Fine-grained, medium-dark gray, dull to slightly silky luster, rounded and smooth from abrasion; likely Mohs hardness 5–6, with no visible cleavage. The uniform texture may also fit a dark sedimentary or metamorphic pebble.
Formation & geological history
Probably eroded from a volcanic or mafic bedrock source and rounded by stream or shoreline transport; its geological age cannot be determined from the photograph. Surface weathering has produced the smooth exterior.
Uses & applications
Commonly used as landscaping gravel, garden stone, aggregate, or a casual collectible; this individual pebble has little commercial value.
Geological facts
Basalt is rich in plagioclase and pyroxene, commonly with minor olivine and magnetite. Its dark color reflects abundant iron- and magnesium-bearing minerals.
Field identification & locations
Look for a dense, fine-grained texture, dark gray-black color, weak magnetic response, and lack of fizz in dilute hydrochloric acid; a streak, hardness, and thin-section test would distinguish basalt from slate, chert, or other dark rocks. Dark rounded pebbles occur widely in stream gravels and glacial deposits.
