igneous
Rounded volcanic rock, likely basaltic andesite
Fine- to medium-grained mafic volcanic rock; probable basalt or basaltic andesite
AI-generated identification · may be incorrectDark brown-gray, rounded and weathered, with a dull to slightly vitreous luster and abundant tiny pale crystals, likely plagioclase, in a fine groundmass. Estimated Mohs hardness 5–6; cleavage is absent at specimen scale, and vesicles or weathering cavities are visible.
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Physical properties
Dark brown-gray, rounded and weathered, with a dull to slightly vitreous luster and abundant tiny pale crystals, likely plagioclase, in a fine groundmass. Estimated Mohs hardness 5–6; cleavage is absent at specimen scale, and vesicles or weathering cavities are visible.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface; the exact age and locality cannot be determined from the photograph. Its smooth shape likely results from stream or shoreline abrasion.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and occasionally decorative stone; ordinary specimens have little jewelry value.
Geological facts
The pale grains may be plagioclase phenocrysts, while the dark matrix may contain pyroxene, olivine, and volcanic glass. Positive identification requires a fresh surface, hand lens, streak, magnetism, and ideally thin-section or chemical analysis.
Field identification & locations
Look for a dense dark color, fine texture, scattered feldspar crystals, and weak attraction to a magnet if magnetite is present; common worldwide in volcanic regions and river gravels.
