igneous
Porphyritic rhyolite
Rhyolite, a silica-rich volcanic rock; likely quartz–feldspar porphyry
AI-generated identification · may be incorrectHardness about 6–7; pink-brown groundmass with conspicuous pale feldspar/quartz phenocrysts and a dark, fine-grained matrix. Dull to slightly glassy luster, irregular fracture, no obvious cleavage as a rock; density typically 2.3–2.7 g/cm³.
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Physical properties
Hardness about 6–7; pink-brown groundmass with conspicuous pale feldspar/quartz phenocrysts and a dark, fine-grained matrix. Dull to slightly glassy luster, irregular fracture, no obvious cleavage as a rock; density typically 2.3–2.7 g/cm³.
Formation & geological history
Formed when viscous, silica-rich lava cooled relatively quickly at or near Earth’s surface, preserving larger crystals formed earlier underground. Its precise geological age cannot be determined from the image; rhyolite occurs from Precambrian to modern volcanic settings.
Uses & applications
Used locally as decorative stone, aggregate, and building or landscaping material; attractive pieces are collected as cabinet specimens.
Geological facts
The large light crystals are phenocrysts, revealing two-stage cooling. The dark speckled portion may represent basaltic inclusion or a weathered darker margin, so exact identification would require a fresh surface and testing.
Field identification & locations
Common in volcanic provinces and continental calderas, including the western United States, Mexico, Iceland, and parts of Europe. Look for abundant quartz/feldspar crystals in a fine pink volcanic matrix; a streak, hardness, and thin-section test can confirm it.
