sedimentary/igneous intermediary

Thunderegg (Ate Agate/Jasper Geode)

Silicon Dioxide (SiO2) within Rhyolite/Perlite matrix

Hardness: 6.5-7 (Mohs scale); Color: Earthy brown and gray exterior with vivid reds, oranges, and deep browns inside; Luster: Vitreous to waxy on internal surfaces, dull on exterior; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 6
Luster: Vitreous to waxy on internal surfaces, dull on exterior

Identified March 21, 2026More sedimentary/igneous intermediary →

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Physical properties

Formation & geological history

Formed during the Tertiary period as gas pockets in rhyolitic volcanic ash beds. Dissolved silica-bearing fluids later seeped into these cavities, crystallizing over thousands of years into bands of agate, jasper, or common opal.

Uses & applications

Primary use is for lapidary work, jewelry (cabochons), and as display specimens for rock collectors.

Geological facts

The Thunderegg is the official state rock of Oregon. According to Native American legend (Warm Springs), these rocks were cast by the 'Thunder Spirits' or 'Thunder Birds' who lived in Mount Hood and Mount Jefferson during volcanic eruptions.

Field identification & locations

Identify in the field by looking for spherical, knobby nodules that look like common rocks but feel unusually heavy for their size. Most commonly found in the Pacific Northwest (USA), Germany, and Australia. Collectors should look for 'knobby' textures on the exterior skin.

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