sedimentary/igneous association

Thunderegg (Agate-filled Geode Node)

Silicon dioxide (SiO2) with various inclusions

Hardness: 6.5-7 Mohs; Color: Tan outer shell with white, grey, and clear translucent banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65

Hardness: 6
Luster: Waxy to vitreous

Identified March 12, 2026More sedimentary/igneous association →

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Physical properties

Formation & geological history

Formed within gas cavities in rhyolitic volcanic ash beds. Over millions of years (often Eocene to Miocene), silica-rich fluids seeped into the hollow nodules, depositing layers of chalcedony and agate.

Uses & applications

Primarily used as a collector specimen, for lapidary work (cabs and polishing), and occasionally in jewelry as ornamental stones.

Geological facts

Thundereggs are the state rock of Oregon. According to Native American legend, they were rocks thrown by the 'Thunder Spirits' living in the Cascade Range peaks.

Field identification & locations

Identify in the field by their rough, cauliflower-like exterior and common presence in rhyolite formations. Found frequently in the Western USA (Oregon, Nevada) and Germany. Collectors look for complete internal filling and distinct banding.

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