igneous
Dark beach-worn pebble
Likely basalt (fine-grained mafic volcanic rock)
AI-generated identification · may be incorrectTypically black to dark gray, dull to subtly glassy, fine-grained, and dense; hardness about 5–6 Mohs, with no visible cleavage and irregular fracture. The rounded shape and surface texture indicate abrasion and weathering; exact identification requires a fresh break or testing.
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Physical properties
Typically black to dark gray, dull to subtly glassy, fine-grained, and dense; hardness about 5–6 Mohs, with no visible cleavage and irregular fracture. The rounded shape and surface texture indicate abrasion and weathering; exact identification requires a fresh break or testing.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. This specimen was likely transported and rounded by waves or glacial action.
Uses & applications
Used as crushed aggregate, road stone, riprap, paving, and occasionally architectural stone; ordinary pebbles have little jewelry value.
Geological facts
Basalt commonly contains plagioclase, pyroxene, and sometimes olivine or magnetite, explaining its dark color and magnetic response. A hand magnet may show weak attraction if magnetite is present.
Field identification & locations
Look for fine texture, high density, dark color, and tiny pale feldspar grains; a streak is usually gray, and dilute acid produces no reaction. Common on volcanic coasts, riverbeds, and glacial deposits.
