igneous
Rhyolite (iron-stained, weathered volcanic rock)
Rhyolite; chiefly quartz and alkali feldspar with minor plagioclase, biotite, and iron oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pink-red, tan, and yellow mottling; dull to weakly waxy luster; fine-grained volcanic texture, possibly with tiny cavities and quartz/feldspar phenocrysts. Cleavage is absent as a rock, and specific gravity is typically about 2.4–2.7.
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Physical properties
Hardness about 6–7 Mohs; pink-red, tan, and yellow mottling; dull to weakly waxy luster; fine-grained volcanic texture, possibly with tiny cavities and quartz/feldspar phenocrysts. Cleavage is absent as a rock, and specific gravity is typically about 2.4–2.7.
Formation & geological history
Forms when silica-rich lava cools rapidly at or near Earth’s surface, commonly with oxidation and hydrothermal alteration producing red and yellow iron staining. Most examples formed during the Cenozoic or older volcanic episodes, depending on locality.
Uses & applications
Used locally as crushed aggregate, building stone, and decorative rock; attractive pieces are collected, though this specimen appears more ornamental than gem-grade.
Geological facts
Rhyolite is the volcanic equivalent of granite and may contain hidden glassy or flow-banded zones. The cavity and coloration could reflect weathering or minor mineralization.
Field identification & locations
Identify it by its light pink-gray color, very fine grain, hardness, and association with volcanic terrain; a streak, acid test, and hand lens help distinguish it from jasper or altered basalt. Common in volcanic regions worldwide, including the western United States, Mexico, Australia, and Europe.
