mineral

Agate (Thunderegg)

Silicon dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Greyish-blue chalcedony core with brown/tan rhyolite exterior; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Hardness: 6
Luster: Waxy to vitreous

Identified June 20, 2026More mineral →

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Physical properties

Formation & geological history

Formed within gas bubbles in volcanic rhyolite ash layers. As the lava cooled, silica-rich fluids seeped into the cavities, solidifying over millions of years into chalcedony or quartz. Most are from the Cenozoic era.

Uses & applications

Used primarily in lapidary arts for jewelry, collecting, and as decorative ornaments or bookends due to their unique banded or filled patterns.

Geological facts

Thundereggs are the state rock of Oregon. According to Native American legend, they were thunder spirits that lived in the peaks of Mount Hood and Mount Jefferson that threw these 'eggs' at each other during storms.

Field identification & locations

Identify in the field by looking for spherical, knobby-textured nodules that look like common rocks but are unusually heavy for their size. Often found in areas of decomposed volcanic ash or pumice.