igneous
Porphyritic basaltic-andesite pebble
Fine-grained volcanic rock, likely plagioclase-phyric basaltic andesite with dark mafic minerals
AI-generated identification · may be incorrectTypically 5–6 Mohs; pale gray-beige matrix with black hornblende/pyroxene and white plagioclase phenocrysts, dull to slightly vitreous luster, massive texture, and no obvious cleavage. The rounded shape indicates water abrasion.
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Physical properties
Typically 5–6 Mohs; pale gray-beige matrix with black hornblende/pyroxene and white plagioclase phenocrysts, dull to slightly vitreous luster, massive texture, and no obvious cleavage. The rounded shape indicates water abrasion.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled at or near Earth’s surface, probably during the Cenozoic or older; its exact age cannot be determined visually. Later erosion transported and rounded it into a pebble.
Uses & applications
Commonly used as aggregate, riprap, landscaping stone, and occasionally as a teaching or decorative specimen; not generally a jewelry gemstone. Collector value is usually minimal unless locality or texture is distinctive.
Geological facts
The contrasting dark patches and lighter matrix reflect porphyritic cooling: larger crystals formed first, followed by rapid solidification of the groundmass. Weathering may produce the tan surface coating.
Field identification & locations
Look for a dense, fine-grained volcanic texture, scattered pale feldspar crystals, dark mafic grains, and a lack of sedimentary layering or visible quartz. Common in volcanic terrains and stream or beach gravels; estimated value is $1–$10 as an ordinary pebble, or $10–$30 with documented locality.
