igneous
Porphyritic basalt or andesite cobble
Fine-grained volcanic rock, likely plagioclase–pyroxene basalt/andesite
AI-generated identification · may be incorrectDark gray-purple, fine-grained matrix with scattered pale angular crystals and weathered light rind; likely Mohs 5–6, dull to weakly vitreous luster, no obvious cleavage, typical igneous density about 2.7–3.0 g/cm³.
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Physical properties
Dark gray-purple, fine-grained matrix with scattered pale angular crystals and weathered light rind; likely Mohs 5–6, dull to weakly vitreous luster, no obvious cleavage, typical igneous density about 2.7–3.0 g/cm³.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, producing a fine matrix around larger early-formed crystals; likely Cenozoic or older, but age cannot be determined from the photograph.
Uses & applications
Commonly used as aggregate, road stone, riprap, and landscaping material; this irregular cobble has modest decorative or educational value rather than gemstone value.
Geological facts
The visible larger crystals indicate porphyritic texture, recording two-stage cooling: slow crystallization underground followed by rapid eruption or exposure.
Field identification & locations
Identify with a hand lens and streak test: hard, nonporous, dark matrix with feldspar-like crystals and no fizz in dilute acid supports basalt/andesite; a fresh surface and thin-section or chemical test are needed for a firm distinction. Common in volcanic regions worldwide.
