igneous
River-worn basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray-green, dense, fine-grained, and dull to subtly silky; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.1. Rounded edges indicate prolonged water abrasion, with reddish patches likely iron-oxide weathering or minor alteration.
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Physical properties
Dark gray-green, dense, fine-grained, and dull to subtly silky; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.1. Rounded edges indicate prolonged water abrasion, with reddish patches likely iron-oxide weathering or minor alteration.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, generally from Cenozoic or older volcanic activity. The pebble was later transported and rounded by a river or beach environment.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, and sometimes dimension stone; attractive polished pieces are occasionally sold as landscaping or collecting specimens.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, pillow lavas, and ocean-floor crust.
Field identification & locations
Identify it by its dark, compact, fine texture, high density, and lack of large crystals; a streak test is usually gray, and it is not strongly attracted to a magnet. Common near volcanic terrains, river gravels, and beaches draining basaltic bedrock.
