igneous
Agate-bearing rhyolite (likely polished/cut specimen)
Silica-rich volcanic rock containing microcrystalline quartz/chalcedony, SiO₂, with feldspar and possible iron oxides
AI-generated identification · may be incorrectHardness about 6.5–7; pink, tan, brown, and gray coloration; waxy to dull luster; very fine-grained matrix with a dense chalcedony-rich band. Fracture is typically conchoidal; no obvious cleavage.
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Physical properties
Hardness about 6.5–7; pink, tan, brown, and gray coloration; waxy to dull luster; very fine-grained matrix with a dense chalcedony-rich band. Fracture is typically conchoidal; no obvious cleavage.
Formation & geological history
Formed when silica-rich volcanic lava cooled and trapped gas cavities later filled by layered chalcedony and quartz. The exact geological age cannot be determined from the photograph; such rhyolites occur from Precambrian to recent volcanic terrains.
Uses & applications
Used as decorative stone, cabochons, bookends, and collecting specimens; massive rhyolite is also used locally as aggregate or building stone.
Geological facts
The reddish-brown coloration commonly comes from iron oxides. Agate banding develops as mineral-rich groundwater repeatedly deposits silica in volcanic cavities.
Field identification & locations
Look for a fine-grained volcanic host, irregular pale chalcedony bands, conchoidal edges, and hardness sufficient to scratch glass. Common in volcanic regions worldwide, especially Brazil, Mexico, the western United States, and Australia.
