igneous
Rounded porphyritic volcanic rock, likely andesite or basalt
Porphyritic andesite–basalt; composition commonly plagioclase, pyroxene, and iron-magnesium minerals
AI-generated identification · may be incorrectDark brown-gray, fine-grained matrix with conspicuous pale mineral crystals and metallic weathering; likely hardness 6–7, dull to subvitreous luster, massive texture, and no obvious cleavage. Its smooth oval shape is from water abrasion rather than crystal form.
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Physical properties
Dark brown-gray, fine-grained matrix with conspicuous pale mineral crystals and metallic weathering; likely hardness 6–7, dull to subvitreous luster, massive texture, and no obvious cleavage. Its smooth oval shape is from water abrasion rather than crystal form.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, after earlier growth of larger crystals underground; likely Cenozoic or older, though age cannot be determined visually. Common in volcanic regions and stream or beach gravels.
Uses & applications
Used locally as aggregate, riprap, paving stone, and decorative stone; an ordinary rounded specimen has little commercial value.
Geological facts
The pale streaks may be feldspar-rich bands, veins, or weathered mineral patches; image-only identification cannot reliably distinguish andesite from basalt or altered volcanic rock.
Field identification & locations
Look for a dense fine matrix, scattered feldspar crystals, dark mafic minerals, and lack of layering or strong foliation; test with a steel file and inspect a fresh broken surface. Common in volcanic belts worldwide, including the Pacific Ring of Fire.
