igneous
Rounded porphyritic volcanic rock, likely andesite
Porphyritic andesite; chiefly plagioclase feldspar, pyroxene and/or hornblende in a fine volcanic groundmass
AI-generated identification · may be incorrectTypically Mohs 5–6, brown to gray with scattered pale feldspar crystals, dull to faintly vitreous luster, and no obvious cleavage as a whole rock. The rounded shape and smooth surface indicate water abrasion, likely as a river or beach cobble.
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Physical properties
Typically Mohs 5–6, brown to gray with scattered pale feldspar crystals, dull to faintly vitreous luster, and no obvious cleavage as a whole rock. The rounded shape and smooth surface indicate water abrasion, likely as a river or beach cobble.
Formation & geological history
Formed when intermediate-composition lava cooled relatively quickly at or near Earth’s surface, preserving larger phenocrysts in a finer matrix; its exact geological age cannot be determined from the photograph. Andesite commonly forms in volcanic arcs above subduction zones.
Uses & applications
Used locally as aggregate, riprap, landscaping stone, and occasionally decorative cobbles; ordinary specimens have little jewelry value.
Geological facts
The mottled brown appearance may reflect iron-oxide weathering, and some pale patches could be altered feldspar or vesicle fillings. Identification is provisional without a fresh surface, hardness test, streak, or thin section.
Field identification & locations
Look for a fine-grained gray-brown matrix with visible feldspar crystals; andesite is common in volcanic mountain belts worldwide, including the Andes, Cascades, Japan, and Indonesia.
