sedimentary
Thunder Egg (Agate filled)
Agate (SiO2) within a Rhyolitic Geode
Hardness: 6.5-7 (Mohs scale), Color: Brown exterior with gray/blue/white banded agate interior, Luster: Waxy to vitreous in the center, dull on the rind, Structure: Microcrystalline quartz (chalcedony), no cleavage, specific gravity ~2.6.
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Brown exterior with gray/blue/white banded agate interior, Luster: Waxy to vitreous in the center, dull on the rind, Structure: Microcrystalline quartz (chalcedony), no cleavage, specific gravity ~2.6.
Formation & geological history
Formed in volcanic ash layers (rhyolite) during the Eocene to Miocene epochs. Silica-rich fluids gas bubbles or cavities, slowly precipitating layers of chalcedony over thousands of years.
Uses & applications
Primarily used as a collector specimen, for lapidary work (slicing and polishing), and occasionally in jewelry for its unique banding patterns.
Geological facts
Thunder eggs are the state rock of Oregon. According to Native American legend, the spirits living in Mt. Hood and Mt. Jefferson would throw these rocks at each other during 'thunder' storms.
Field identification & locations
Identify in the field by looking for spherical, knobby nodules with a rough, 'cauliflower' like rhyolite crust. They are common in the Pacific Northwest (Oregon, Washington) and parts of Germany and Australia.
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