igneous
Rounded porphyritic volcanic rock, likely andesite or rhyolite
Fine-grained silicate rock; likely plagioclase- and quartz/feldspar-bearing andesite–rhyolite
AI-generated identification · may be incorrectDark gray-brown, reddish weathering, dull to slightly silky luster, and massive texture with possible porphyritic crystals; estimated Mohs hardness 5.5–7, with no obvious cleavage. The smooth shape appears naturally water-worn or polished.
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Physical properties
Dark gray-brown, reddish weathering, dull to slightly silky luster, and massive texture with possible porphyritic crystals; estimated Mohs hardness 5.5–7, with no obvious cleavage. The smooth shape appears naturally water-worn or polished.
Formation & geological history
Formed when silica-rich magma cooled, probably as an ancient lava flow or shallow intrusion; its exact geological age and composition cannot be determined from the photograph. Rounded by stream, beach, or glacial transport and abrasion.
Uses & applications
Commonly used as aggregate, riprap, landscaping stone, and occasionally decorative material; this specimen has modest collector value unless testing reveals unusual mineralization.
Geological facts
Red patches may reflect iron oxidation, while pale areas may be feldspar, quartz, or weathered alteration minerals. A streak, hardness, acid, and thin-section test would distinguish andesite from rhyolite or altered basalt.
Field identification & locations
Likely locations include volcanic regions worldwide, such as the Cascades, Andes, Japan, Iceland, and Mediterranean belts. Look for fine groundmass, scattered crystals, conchoidal-to-uneven fracture, and lack of visible sedimentary layering.
