igneous
Porphyritic rhyolite (likely weathered volcanic rock)
Rhyolite, a volcanic equivalent of granite; chiefly quartz and alkali feldspar in a fine-grained groundmass
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pinkish-brown and cream with conspicuous pale or reddish feldspar phenocrysts, dull to slightly vitreous luster, and no obvious cleavage. The rounded, weathered surface obscures its original texture.
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Physical properties
Hardness about 6–7 Mohs; pinkish-brown and cream with conspicuous pale or reddish feldspar phenocrysts, dull to slightly vitreous luster, and no obvious cleavage. The rounded, weathered surface obscures its original texture.
Formation & geological history
Forms when silica-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic domes and flows. It may range from Precambrian to recent in age, depending on locality.
Uses & applications
Used locally as aggregate, road stone, and building or facing stone; attractive pieces are collected as cabinet specimens, though this example appears modestly valuable.
Geological facts
The large crystals formed during slow cooling in a magma chamber before the remaining melt erupted and cooled rapidly, producing the fine matrix.
Field identification & locations
Identify it by its light felsic color, abundant embedded feldspar crystals, and lack of layering or foliation; confirm with a streak, hardness, and ideally thin-section or chemical testing. Common in volcanic regions worldwide.
