igneous
Weathered brown-gray igneous rock, likely basaltic andesite
Porphyritic basaltic andesite; probable plagioclase–pyroxene volcanic rock
AI-generated identification · may be incorrectDark gray to brown, fine-grained matrix with conspicuous pale weathered mineral patches and possible plagioclase crystals; dull to slightly earthy luster, no obvious cleavage, typically Mohs 5–6, specific gravity about 2.7–3.0. The rounded, smoothed shape indicates water or abrasion weathering.
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Physical properties
Dark gray to brown, fine-grained matrix with conspicuous pale weathered mineral patches and possible plagioclase crystals; dull to slightly earthy luster, no obvious cleavage, typically Mohs 5–6, specific gravity about 2.7–3.0. The rounded, smoothed shape indicates water or abrasion weathering.
Formation & geological history
Formed when intermediate-to-mafic lava cooled relatively quickly at or near Earth’s surface, likely during the Cenozoic or older depending on locality; the exact age cannot be determined from the image. Brown coloration reflects iron oxidation and surface weathering.
Uses & applications
Commonly used as crushed aggregate, roadstone, riprap, and occasionally decorative stone; this ordinary specimen has little gem value.
Geological facts
The light areas may be altered plagioclase, quartz, or a weathering rind, but mineral identification requires a fresh surface or testing. It may also be a weathered basalt, andesite, or volcanic breccia fragment.
Field identification & locations
Look for a dark fine-grained groundmass, pale feldspar phenocrysts, high density, and lack of fizz with dilute hydrochloric acid. Common in volcanic terrains and stream or beach gravels worldwide.
