igneous
Thundereggs (agate-filled geodes)
Silica-rich volcanic nodules containing chalcedony (microcrystalline quartz, SiO₂), quartz, and commonly zeolites or calcite
AI-generated identification · may be incorrectTypically rounded, brown volcanic-rock rinds with cavities lined by gray, white, blue, or reddish banded chalcedony; hardness about 6.5–7 Mohs, waxy to vitreous luster, no cleavage, and specific gravity near 2.6. The cut faces show radiating or concentric agate patterns and druzy quartz.
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Physical properties
Typically rounded, brown volcanic-rock rinds with cavities lined by gray, white, blue, or reddish banded chalcedony; hardness about 6.5–7 Mohs, waxy to vitreous luster, no cleavage, and specific gravity near 2.6. The cut faces show radiating or concentric agate patterns and druzy quartz.
Formation & geological history
Formed when gas bubbles in rhyolitic volcanic lava became sealed, then filled progressively by silica-rich groundwater. They commonly formed in Cenozoic volcanic terrains, though exact age depends on locality.
Uses & applications
Cut and polished as lapidary specimens, bookends, display pieces, cabochons, and jewelry; rarely used industrially because specimens are valued mainly for appearance.
Geological facts
The outer material is usually rhyolite or welded volcanic tuff, while the interior is an agate geode. Oregon’s state rock is the thunderegg, especially those from central Oregon.
Field identification & locations
Identify by a hard volcanic rind, irregular rounded shape, and hollow or partially filled interior with concentric chalcedony bands; common sources include Oregon, Idaho, Utah, Mexico, Brazil, and Australia. A definitive locality requires labels or testing.
