sedimentary
Thunderegg (Agate or Quartz Geode)
Silica-filled rhyolitic nodule (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: Outer shell is usually tan/grey rhyolite, interior can be white, pink, or translucent. Luster: Dull/earthy exterior, vitreous/waxy interior. Crystal structure: Cryptocrystalline to microcrystalline.
- Hardness
- 6
- Luster
- Dull/earthy exterior, vitreous/waxy interior
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Outer shell is usually tan/grey rhyolite, interior can be white, pink, or translucent. Luster: Dull/earthy exterior, vitreous/waxy interior. Crystal structure: Cryptocrystalline to microcrystalline.
Formation & geological history
Formed in rhyolitic lava flows when gas bubbles fill with silica-rich fluids over millions of years (Cenozoic era). Many are found in volcanic ash and tuff beds.
Uses & applications
Primarily used as lapidary material, decorative display pieces, and for mineral collecting.
Geological facts
Thundereggs are the state rock of Oregon (USA). Native American legend says they were thrown by the Thunder Spirits during storms at the peaks of Mt. Hood and Mt. Jefferson.
Field identification & locations
Identify by their rough, cauliflower-like exterior and 'star' or 'cross' shaped core when cut. Common in the Western US, Germany, and Australia. Collectors should look for heavy, round nodules in volcanic regions.
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