igneous
Polished rhyolite (likely orbicular or flow-banded rhyolite)
Silica-rich volcanic rock, typically quartz–alkali feldspar–plagioclase with volcanic glass and iron oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, cream, rust-red, and brown bands with a matte-to-waxy polished luster. Fine-grained, commonly porphyritic or flow-banded; cleavage is absent and fracture is irregular to conchoidal.
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Physical properties
Hardness about 6–7 Mohs; gray, cream, rust-red, and brown bands with a matte-to-waxy polished luster. Fine-grained, commonly porphyritic or flow-banded; cleavage is absent and fracture is irregular to conchoidal.
Formation & geological history
Formed from viscous rhyolitic lava or welded volcanic ash that cooled rapidly, preserving flow banding and altered mineral-rich zones. Most examples are Cenozoic or younger, though rhyolite occurs throughout geological time.
Uses & applications
Used as a decorative stone, lapidary material, cabochons, and collectible polished specimens; rarely used as aggregate or building stone.
Geological facts
The red and yellow coloration commonly results from hematite, goethite, or other iron-oxide alteration. Rounded surfaces indicate tumbling or natural water abrasion.
Field identification & locations
Identify it by its very fine texture, pale silica-rich body, streaky flow bands, and lack of visible large crystals; a steel file should barely scratch it. Common in volcanic regions such as the western United States, Mexico, New Zealand, and Iceland.
