mineral
Thunder Egg (Agate Core)
Cryptocrystalline Quartz (SiO2) in Rhyolite/Perlite Matrix
Hardness: 6.5-7 on Mohs scale; Color: Tan/brown host rock with white, grey, and bluish agate banding; Luster: Vitreous to waxy in agate zone, dull in outer matrix; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None
- Hardness
- 6
- Luster
- Vitreous to waxy in agate zone, dull in outer matrix
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan/brown host rock with white, grey, and bluish agate banding; Luster: Vitreous to waxy in agate zone, dull in outer matrix; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None
Formation & geological history
Formed in silica-rich volcanic lava flows (rhyolite). Gas bubbles created cavities that filled over time with silica-rich water solutions, precipitating layers of chalcedony, jasper, and agate.
Uses & applications
Popular as lapidary material, display specimens, paperweights, cut and polished slices, and jewelry cabs.
Geological facts
Thunder eggs are the official state rock of Oregon. They appear as nondescript, round rough rocks on the outside, but reveal intricate agate or opal centers when cut open.
Field identification & locations
Look for rough, bumpy, spherical or egg-shaped nodules in volcanic regions (especially Oregon, New Mexico, and Mexico). Cutting with a diamond saw reveals the hidden agate interior.
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