igneous
Weathered porphyritic volcanic rock
Likely porphyritic andesite or basalt; exact identification requires a fresh surface and testing
AI-generated identification · may be incorrectDark gray-brown, fine-grained groundmass with scattered pale feldspar-rich patches and abundant lichen. Hardness approximately 6–7, dull to faintly vitreous luster, poor or absent cleavage, and typical specific gravity around 2.6–2.9.
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Physical properties
Dark gray-brown, fine-grained groundmass with scattered pale feldspar-rich patches and abundant lichen. Hardness approximately 6–7, dull to faintly vitreous luster, poor or absent cleavage, and typical specific gravity around 2.6–2.9.
Formation & geological history
An extrusive igneous rock formed when lava cooled at or near Earth’s surface, producing larger feldspar crystals in a finer matrix; commonly Cenozoic to older, depending on locality. The surface is strongly weathered and colonized by lichen.
Uses & applications
Useful as local aggregate, road base, riprap, and occasionally dimension stone; not generally valuable as a gemstone. Rough field specimens usually have minimal commercial value.
Geological facts
The pale blotches may be feldspar phenocrysts, weathering products, or lichen rather than distinct minerals. A fresh broken face would best distinguish andesite from basalt.
Field identification & locations
Common in volcanic terrains, especially the Andes, Cascades, Japan, Iceland, and island arcs. Look for a fine matrix with embedded crystals; a streak test, magnet test, hand lens, and hardness test can refine identification.
