igneous
Weathered basalt (vesicular volcanic rock)
Basalt, chiefly plagioclase feldspar, clinopyroxene, and olivine; minor iron oxides
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to slightly earthy luster, with small weathered vesicles and possible iron staining; hardness about 5–6 Mohs, no visible cleavage, typically dense and tough.
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Physical properties
Dark gray to brown, fine-grained, dull to slightly earthy luster, with small weathered vesicles and possible iron staining; hardness about 5–6 Mohs, no visible cleavage, typically dense and tough.
Formation & geological history
An extrusive igneous rock formed when low-silica lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. The tan coating reflects surface weathering and oxidation.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and building stone; attractive polished pieces are used decoratively, but this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and seafloor pillow lavas.
Field identification & locations
Identify it by its dark fine-grained body, heavy feel, sparse vesicles, and lack of obvious quartz or large crystals; common worldwide in volcanic regions. The image alone cannot exclude a weathered andesite or other mafic rock.
