igneous
Polished porphyritic volcanic pebble
Likely porphyritic basalt or andesite; exact identification requires a fresh surface and mineral tests
AI-generated identification · may be incorrectDark gray-brown, fine-grained matrix with scattered pale mineral grains, rounded and polished; typical hardness about 6–7 Mohs, dull to faintly vitreous luster, and no obvious cleavage. The pale spots may be feldspar or altered volcanic minerals.
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Physical properties
Dark gray-brown, fine-grained matrix with scattered pale mineral grains, rounded and polished; typical hardness about 6–7 Mohs, dull to faintly vitreous luster, and no obvious cleavage. The pale spots may be feldspar or altered volcanic minerals.
Formation & geological history
Formed when silica-poor to intermediate magma cooled, with larger crystals developing before rapid final cooling at or near the surface; commonly Cenozoic to older, depending on source locality. Its smooth shape reflects water or artificial tumbling/polishing.
Uses & applications
Suitable as a decorative pebble, cabochon, paperweight, or teaching specimen; not generally valuable for construction in this small polished form.
Geological facts
The contrasting large crystals in a fine groundmass are called porphyritic texture, recording two-stage cooling. The image alone cannot reliably distinguish basalt from andesite.
Field identification & locations
Look for a fine dark groundmass, visible feldspar phenocrysts, dense weight, and lack of vesicles; a streak, magnet test, hardness test, and fresh edge would help. Similar pebbles occur in volcanic river gravels worldwide.
