mineral

Thunderegg

Agate-filled Rhyolitic Nodule (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Dark brown to black agate core with a tan-colored rhyolite shell; Luster: Vitreous to waxy core, earthy shell; Crystal Structure: Cryptocrystalline; Cleavage: None

Hardness: 6
Luster: Vitreous to waxy core, earthy shell

Identified May 13, 2026More mineral →

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Physical properties

Formation & geological history

Formed in gas pockets within rhyolitic volcanic ash beds. Silica-rich fluids seeped into these cavities over millions of years, precipitating layers of chalcedony or agate. Most are found in beds dated to the Eocene or Miocene epochs.

Uses & applications

Primarily used for lapidary work, jewelry (cabs), and as a popular collector specimen for display.

Geological facts

Thundereggs are the state rock of Oregon. According to Native American legend, they were the eggs thrown by the thunderbirds that lived in the peaks of Mt. Hood and Mt. Jefferson.

Field identification & locations

Identify by looking for spherical, rough nodules with a star-shaped or irregular agate core when cut. Commonly found in Oregon, New Mexico, and Germany. Collectors look for unique banding or 'moss' inclusions in the center.