igneous
Water-worn basalt pebble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark brown-black, very fine-grained, massive, and dull to weakly vitreous when wet or polished; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate prolonged abrasion by water or sediment.
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Physical properties
Dark brown-black, very fine-grained, massive, and dull to weakly vitreous when wet or polished; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate prolonged abrasion by water or sediment.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. This specimen was likely transported and rounded in a stream, beach, or glacial deposit.
Uses & applications
Basalt is widely used as crushed aggregate, roadstone, riprap, railway ballast, and dimension stone; attractive polished pieces are used decoratively. This small pebble has little commercial value but may be collectible as a locality specimen.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and is chemically equivalent to gabbro, its slowly cooled intrusive counterpart.
Field identification & locations
Identify it by its uniform dark color, fine texture, high density, and lack of layering or obvious large crystals; a streak, magnet test, and fresh break can help distinguish it from slag or iron-rich sedimentary rocks. Common worldwide, especially in volcanic regions and river or beach gravels.
