igneous
Porphyritic volcanic rock, likely andesite or basalt with weathered feldspar phenocrysts
Porphyritic andesite–basalt; chiefly plagioclase feldspar and pyroxene, with possible olivine or iron oxides
AI-generated identification · may be incorrectDark gray, fine-grained groundmass with conspicuous tan-brown, weathered crystals or vesicle fillings; dull to weakly vitreous luster, massive texture, no obvious cleavage, and approximate hardness 5–6. Rounded edges indicate stream or beach abrasion.
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Physical properties
Dark gray, fine-grained groundmass with conspicuous tan-brown, weathered crystals or vesicle fillings; dull to weakly vitreous luster, massive texture, no obvious cleavage, and approximate hardness 5–6. Rounded edges indicate stream or beach abrasion.
Formation & geological history
Formed when silica-poor to intermediate lava cooled rapidly at or near Earth’s surface, preserving larger crystals formed earlier underground; commonly Cenozoic or younger, though age cannot be determined visually.
Uses & applications
Used locally as aggregate, roadstone, riprap, and landscaping material; generally little jewelry or collector value unless crystals or alteration are unusually attractive.
Geological facts
The tan patches may be altered feldspar, olivine, or amygdules filled by secondary minerals rather than true fossils. A fresh broken surface would best distinguish basalt from andesite by color and mineral abundance.
Field identification & locations
Common in volcanic terrains and glacial or stream deposits. Check for fine crystals with a hand lens, test hardness against glass, and inspect a fresh fracture; acid reaction would indicate carbonate filling rather than the host rock.
