igneous
Polished basalt pebble
Basalt (fine-grained mafic volcanic rock; commonly plagioclase + pyroxene ± olivine and iron oxides)
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark brown to black with rusty iron-oxide staining, fine-grained texture, and subdued vitreous-to-dull luster. Typically massive, with no visible cleavage or large crystals; the smooth surface indicates natural abrasion or polishing.
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Physical properties
Hardness about 6–7 Mohs; dark brown to black with rusty iron-oxide staining, fine-grained texture, and subdued vitreous-to-dull luster. Typically massive, with no visible cleavage or large crystals; the smooth surface indicates natural abrasion or polishing.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Individual specimens may range from ancient Precambrian rocks to geologically recent volcanic material.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, construction blocks, and occasionally as polished decorative stone or a worry stone.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and iron oxides; reddish-brown areas commonly result from weathering. It may be magnetic if it contains appreciable magnetite.
Field identification & locations
Identify it by its high density, dark fine-grained appearance, hardness, and lack of obvious crystals; a streak test is usually gray-black, and acid produces no reaction. Common in volcanic and glacial alluvial settings worldwide.
