igneous
Thunderegg (specifically a Jasper/Agate-filled Geode nodule)
Silicon Dioxide (SiO2) within Rhyolitic Matrix
Hardness: 6.5-7 Mohs; Color: Tan/gray matrix with tan, mahogany, and black interior jasper; Luster: Waxy to vitreous (interior), Dull (exterior); Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Luster
- Waxy to vitreous (interior), Dull (exterior)
Identified More igneous →
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Physical properties
Hardness: 6.5-7 Mohs; Color: Tan/gray matrix with tan, mahogany, and black interior jasper; Luster: Waxy to vitreous (interior), Dull (exterior); Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed in gas pockets (vesicles) within rhyolitic volcanic ash or lava flows. Over time, silica-rich groundwater percolates through the rock, depositing layers of chalcedony, jasper, or opal. These typically date to the Cenozoic era.
Uses & applications
Used primarily as lapidary material for jewelry (cabochons), paperweights, and as popular display specimens for mineral collectors.
Geological facts
Thundereggs are the state rock of Oregon, USA. Legend has it they were thrown by thunder spirits (Thunderbirds) during storms. Unlike regular geodes, they are usually solid rather than hollow.
Field identification & locations
Identify by their rough, cauliflower-like brownish-gray exterior and star-shaped or irregular interior patterns when cut. Commonly found in Oregon, Idaho, and Germany. Collectors look for high-contrast colors and unique interior geometry.
