igneous
Dark gray river pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; identification is approximate from the image)
AI-generated identification · may be incorrectDull to weakly lustrous, dark gray-purple, fine-grained and rounded by abrasion; composed mainly of plagioclase and pyroxene, commonly with olivine. Mohs hardness is about 6; crystals are too small to distinguish here.
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Physical properties
Dull to weakly lustrous, dark gray-purple, fine-grained and rounded by abrasion; composed mainly of plagioclase and pyroxene, commonly with olivine. Mohs hardness is about 6; crystals are too small to distinguish here.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools at or near the surface. Its age cannot be determined from appearance; basalt occurs from ancient flows to recent volcanic deposits.
Uses & applications
Commonly used as crushed aggregate, road stone, and construction material; rounded pebbles are also used in landscaping and collections.
Geological facts
The smooth shape is consistent with prolonged transport and abrasion in a river or along a shore.
Field identification & locations
Look for a dense, fine-grained texture and dark color; a fresh broken surface may reveal tiny crystals. Basalt is widespread in volcanic regions and river gravels sourced from them.
