igneous
Basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained, moderately heavy, with a dull to weakly subvitreous luster and no obvious cleavage; typical hardness is about 5–6 Mohs. The rounded shape and scattered lighter grains suggest water-worn basalt, possibly with vesicles or weathered mineral inclusions.
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Physical properties
Dark gray-black, fine-grained, moderately heavy, with a dull to weakly subvitreous luster and no obvious cleavage; typical hardness is about 5–6 Mohs. The rounded shape and scattered lighter grains suggest water-worn basalt, possibly with vesicles or weathered mineral inclusions.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the specimen was likely rounded in a stream, beach, or glacial deposit.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, building stone, and landscape rock; attractive polished pieces are used decoratively but have little gemstone value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Identify it by its black color, fine texture, high density, and lack of visible large crystals; a streak, magnetism test, and close inspection for vesicles can help distinguish it from slag or other dark rocks. Common locations include volcanic regions and many river or beach gravels.
