igneous
Weathered porphyritic volcanic rock
Likely andesite or dacite, with feldspar phenocrysts in a fine-grained groundmass
AI-generated identification · may be incorrectGray-brown, mottled, and dull to slightly earthy; visible pale crystal grains suggest porphyritic texture. Likely hardness about 6–7 Mohs, with no obvious cleavage on the rounded surface; exact density and mineralogy require testing.
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Physical properties
Gray-brown, mottled, and dull to slightly earthy; visible pale crystal grains suggest porphyritic texture. Likely hardness about 6–7 Mohs, with no obvious cleavage on the rounded surface; exact density and mineralogy require testing.
Formation & geological history
Formed when intermediate volcanic magma cooled, typically in a lava flow or shallow intrusion; the specimen is likely strongly weathered and rounded by erosion. Its geological age cannot be determined from the photograph.
Uses & applications
Common andesite/dacite is used as aggregate, road stone, riprap, and occasionally building stone; this small piece has mainly educational or modest collecting value.
Geological facts
The larger light crystals are probably plagioclase feldspar, while darker grains may be pyroxene, amphibole, or altered biotite. A streak, hardness, magnetism, and fresh-surface inspection would improve identification.
Field identification & locations
Common in volcanic mountain belts and island arcs, including the Andes, Cascades, Japan, and Indonesia. In the field, distinguish it from granite by its finer groundmass and from basalt by its generally lighter color; value is typically $1–$10 for an ordinary hand specimen.
