igneous
Rounded dark-gray cobble, likely basalt
Basalt (fine-grained mafic volcanic rock; identification is provisional from the image)
AI-generated identification · may be incorrectGray-black, fine-grained, with a dull to weakly lustrous surface and rounded shape. Basalt is typically Mohs 5–6, composed mainly of plagioclase and pyroxene, sometimes olivine; crystals are too small to see unaided.
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Physical properties
Gray-black, fine-grained, with a dull to weakly lustrous surface and rounded shape. Basalt is typically Mohs 5–6, composed mainly of plagioclase and pyroxene, sometimes olivine; crystals are too small to see unaided.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface; basalt is produced in settings ranging from ocean floors to volcanic fields and spans much of geological time.
Uses & applications
Basalt is widely used as crushed aggregate, road stone, and construction material; attractive cobbles may be kept as display or collecting specimens.
Geological facts
The rounded outline suggests transport and abrasion by water or other sedimentary processes, but does not establish the rock’s origin.
Field identification & locations
Look for a dense, fine-grained texture and dark color; confirm with a fresh surface and close inspection for visible crystals or vesicles. Basalt occurs worldwide, especially in volcanic regions and river or beach gravels.
