igneous
Polished porphyritic volcanic rock (likely rhyolite or andesite)
Porphyritic rhyolite–andesite; fine volcanic groundmass with abundant pale feldspar phenocrysts and possible vesicles
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark reddish-brown, rounded and polished, with dull to subtly vitreous luster. Pale cream-gray inclusions are likely feldspar crystals or altered mineral patches; no obvious cleavage is visible.
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Physical properties
Hardness about 6–7 Mohs; dark reddish-brown, rounded and polished, with dull to subtly vitreous luster. Pale cream-gray inclusions are likely feldspar crystals or altered mineral patches; no obvious cleavage is visible.
Formation & geological history
Formed when silica-rich to intermediate lava cooled relatively quickly at or near Earth’s surface, preserving larger crystals in a fine matrix; likely Cenozoic or older, though age cannot be determined visually. Its rounded form indicates stream, beach, or deliberate tumbling/polishing.
Uses & applications
Used mainly as a lapidary specimen, decorative stone, or collector’s piece; comparable volcanic rock is used for aggregate, paving, and architectural stone.
Geological facts
The contrasting spots are phenocrysts or weathered vesicle fillings, producing a porphyritic texture. Exact identification requires a thin section or tests for streak, density, and mineral composition.
Field identification & locations
Look for a fine-grained volcanic matrix containing conspicuous feldspar crystals, unlike coarse granite; test hardness against glass and inspect any cavities for mineral coatings. Common sources include volcanic regions worldwide, especially the western United States, Mexico, and Europe.
