igneous
Basalt pebble
Basalt; primarily plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, typically dull to slightly vitreous; Mohs hardness about 6, no visible cleavage in the whole rock, and specific gravity roughly 2.8–3.0. The pictured pebble appears rounded and weathered, with possible vesicular or glassy areas.
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Physical properties
Fine-grained, dark gray to black, typically dull to slightly vitreous; Mohs hardness about 6, no visible cleavage in the whole rock, and specific gravity roughly 2.8–3.0. The pictured pebble appears rounded and weathered, with possible vesicular or glassy areas.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, generally in volcanic settings from Precambrian to modern age. It commonly occurs in lava flows, volcanic fields, and river or beach gravels.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive dense pieces are also collected or polished. This small common pebble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms at mid-ocean ridges and hotspots. Some basalts contain magnetic minerals and can weakly attract a magnet.
Field identification & locations
Identify it by its dark color, fine texture, substantial weight, and lack of obvious large crystals; a streak, magnet test, and hand lens help distinguish it from slag or dark sedimentary rocks. Common worldwide, especially in volcanic regions and stream deposits.
