igneous
Tumbled basalt (vesicular volcanic stone)
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically dark brown to black with a dull to submetallic sheen; Mohs hardness about 5–6, fine-grained, and commonly vesicular (pore-bearing). The rounded shape and glossy surface indicate tumbling or polishing; no visible cleavage.
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Physical properties
Typically dark brown to black with a dull to submetallic sheen; Mohs hardness about 5–6, fine-grained, and commonly vesicular (pore-bearing). The rounded shape and glossy surface indicate tumbling or polishing; no visible cleavage.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, likely during the Cenozoic or younger volcanic activity. Its pores formed from gas bubbles trapped during solidification.
Uses & applications
Used as landscaping stone, aggregate, road material, ballast, and decorative tumbled stone; occasionally sold in meditation or craft collections. Common specimens generally have modest commercial value.
Geological facts
The numerous small cavities are vesicles, not fossil structures; weathering and polishing can produce the attractive brown-black surface. Some dense basalt can resemble iron-rich meteorites, but basalt is usually lighter and more porous.
Field identification & locations
Common in volcanic regions, oceanic crust, Hawaii, Iceland, India, and the Pacific Northwest. A magnet may attract it weakly; a fresh edge typically reveals a dark, fine-grained interior, while strong vesiculation helps distinguish it from meteorite material.
