igneous
Porphyritic volcanic rock, likely andesite
Porphyritic andesite; mainly plagioclase feldspar and volcanic groundmass, with possible hornblende or pyroxene
AI-generated identification · may be incorrectDark gray-brown, fine-grained groundmass containing abundant pale, blocky feldspar phenocrysts; generally dull to slightly vitreous luster, no obvious cleavage as a whole, and estimated Mohs hardness 5–6. The specimen appears weathered and irregularly fractured.
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Physical properties
Dark gray-brown, fine-grained groundmass containing abundant pale, blocky feldspar phenocrysts; generally dull to slightly vitreous luster, no obvious cleavage as a whole, and estimated Mohs hardness 5–6. The specimen appears weathered and irregularly fractured.
Formation & geological history
Formed when intermediate-composition lava cooled relatively quickly at or near Earth’s surface, preserving larger crystals that formed earlier underground. Andesite is mainly Cenozoic to modern in volcanic arcs, though older examples occur worldwide.
Uses & applications
Used locally as crushed aggregate, road material, riprap, and building stone; attractive polished pieces may be collected, but it is not normally a gemstone.
Geological facts
The contrasting large feldspar crystals and fine matrix are diagnostic of porphyritic texture. Exact identification would benefit from a fresh surface, streak, density, and thin-section or chemical analysis.
Field identification & locations
Common in volcanic mountain belts, especially the Andes, Cascades, Japan, Indonesia, and New Zealand. Field identification: hard, dense, non-layered volcanic rock with conspicuous pale feldspar crystals; distinguish from granite by its much finer matrix.
