igneous
Vesicular basalt (likely weathered scoriaceous basalt)
Basalt, primarily plagioclase feldspar, pyroxene, and iron-rich oxides; approximate composition 45–52% SiO₂
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly metallic from weathering, with numerous small vesicles and rusty iron-oxide staining. Hardness about 5–6 Mohs; no obvious cleavage; typically dense but porous, with specific gravity around 2.7–3.0.
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Physical properties
Dark gray to black, fine-grained, dull to slightly metallic from weathering, with numerous small vesicles and rusty iron-oxide staining. Hardness about 5–6 Mohs; no obvious cleavage; typically dense but porous, with specific gravity around 2.7–3.0.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near the surface, trapping gas bubbles; commonly Cenozoic to modern in volcanic regions. The brown-gold patches likely represent weathered iron minerals or secondary coatings.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a hand specimen; generally not used as a gemstone.
Geological facts
Vesicles record gas escape during eruption, while scoria is the more highly vesicular, lightweight variety. The specimen’s rounded shape may result from stream or surface weathering.
Field identification & locations
Identify by its black fine-grained matrix, vesicles, heavy feel, and lack of visible large crystals; common near lava flows, volcanic cinder cones, and basaltic beaches. A magnet test may show attraction from magnetite.
