igneous
Weathered porphyritic volcanic rock
Likely porphyritic andesite or dacite, with plagioclase phenocrysts in a fine-grained volcanic groundmass
AI-generated identification · may be incorrectBrown-gray, fine-grained matrix with conspicuous pale, blocky crystals and reddish iron-oxide weathering; likely hardness 5.5–6.5 Mohs, dull to slightly vitreous luster, poor or absent cleavage as a whole, and specific gravity about 2.5–2.8.
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Physical properties
Brown-gray, fine-grained matrix with conspicuous pale, blocky crystals and reddish iron-oxide weathering; likely hardness 5.5–6.5 Mohs, dull to slightly vitreous luster, poor or absent cleavage as a whole, and specific gravity about 2.5–2.8.
Formation & geological history
Formed when viscous lava cooled relatively quickly at or near Earth’s surface, preserving larger crystals formed earlier underground; typically Cenozoic or older volcanic material, though its exact age cannot be determined from the image.
Uses & applications
Used locally as aggregate, riprap, or landscaping stone; attractive pieces may be collected, but it is generally not a gemstone or high-value building stone.
Geological facts
The pale inclusions are probably plagioclase feldspar phenocrysts, while the dark groundmass may contain pyroxene, amphibole, and volcanic glass. Surface oxidation produces the reddish-brown coloration.
Field identification & locations
Common in volcanic mountain belts and lava-flow terrains worldwide. Field confirmation requires a fresh break, hand lens, streak, and ideally thin-section or chemical analysis; abundant quartz would favor dacite, while darker composition favors andesite.
