igneous
Polished vesicular basalt (likely iron-rich volcanic rock)
Basalt, predominantly plagioclase–pyroxene–olivine; surface may include iron-oxide minerals or metallic-looking infill
AI-generated identification · may be incorrectDark gray to black, fine-grained and locally vesicular, with glossy polished luster and irregular cavities containing brown, orange, gray, and pale mineral fillings. Hardness about 5.5–6.5 Mohs; no obvious cleavage; typically dense, though vesicles reduce specific gravity to roughly 2.7–3.1.
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Physical properties
Dark gray to black, fine-grained and locally vesicular, with glossy polished luster and irregular cavities containing brown, orange, gray, and pale mineral fillings. Hardness about 5.5–6.5 Mohs; no obvious cleavage; typically dense, though vesicles reduce specific gravity to roughly 2.7–3.1.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, probably in the Cenozoic or younger unless sourced from an older volcanic province. Vesicles formed from trapped gas and later became lined or filled by secondary minerals during groundwater alteration.
Uses & applications
Used as an ornamental or lapidary specimen, aggregate, road stone, and construction material; attractive polished pieces are collected rather than used as gemstones.
Geological facts
The rounded shape and artificial gloss indicate tumbling or polishing. The orange-brown patches are commonly iron oxides or altered olivine, while pale cavity fillings may be calcite, zeolites, or silica; exact identification requires testing.
Field identification & locations
Identify by its black fine-grained matrix, high density, vesicles, and lack of visible large crystals; a streak, hardness, acid, and magnet test can distinguish basalt from slag or meteorite. Common in volcanic regions worldwide, including Hawaii, Iceland, India, and the western United States.
