igneous
Weathered porphyritic volcanic rock (likely andesite or basalt)
Aphanitic to porphyritic andesite–basalt; composition commonly plagioclase, pyroxene, and minor iron oxides
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; brown-gray weathered surface, dull to slightly earthy luster, dense fine-grained matrix with possible feldspar phenocrysts, and no obvious cleavage. The rounded form and iron staining indicate strong abrasion and oxidation.
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Physical properties
Hardness about 5.5–6.5 Mohs; brown-gray weathered surface, dull to slightly earthy luster, dense fine-grained matrix with possible feldspar phenocrysts, and no obvious cleavage. The rounded form and iron staining indicate strong abrasion and oxidation.
Formation & geological history
An extrusive volcanic rock formed when lava cooled rapidly at or near Earth’s surface, likely during a Cenozoic volcanic episode; exact age and chemistry require thin-section or laboratory analysis.
Uses & applications
Primarily a natural beach or stream cobble with little commercial value; similar andesite and basalt are used as aggregate, roadstone, riprap, and landscaping material.
Geological facts
Its tan-brown exterior is weathering rind rather than necessarily the original rock color. The dark, rough central patch may represent vesicular, altered, or iron-rich material.
Field identification & locations
Common in volcanic terrains and downstream beaches or gravel bars; identify by density, fine texture, gray-black fresh surfaces, and lack of visible quartz. A fresh fracture, streak, acid test, and hardness test would improve identification.
