sedimentary
Thunderegg
Rhyolite nodule with chalcedony/agate core
AI-generated identification · may be incorrectHardness of 7 on Mohs scale, earthy exterior with crystalline quartz or agate core, vitreous luster, cryptocrystalline structure, no cleavage.
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Physical properties
Hardness of 7 on Mohs scale, earthy exterior with crystalline quartz or agate core, vitreous luster, cryptocrystalline structure, no cleavage.
Formation & geological history
Formed in silica-rich volcanic ash layers during the Tertiary period when gas pockets filled with hydrothermal fluids.
Uses & applications
Cut and polished as decorative lapidary items, collector specimens, and occasionally used in jewelry.
Geological facts
Thundereggs are the state rock of Oregon and often contain intricate patterns of agate, jasper, or opal.
Field identification & locations
Identify by spherical or nodular exterior with a star-shaped or solid quartz-filled core when sliced open.
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