Thunderegg, Photo by Reid GarrisonRice Museum of Rocks and Minerals
State Rock of Oregon
While thundereggs can be found all over the world, there are a number of beds in Central and Eastern Oregon. On March 29, 1965 thundereggs were declared Oregon’s official state rock!
What is a Thunderegg?
A thunderegg is a bumpy, spherical shell of volcanic rock with an internal, cracked cavity filled with another material, usually agate.
Meatball, an uncut thunderegg, can be found (and touched) in the Northwest Gallery of the Rice Museum.
Thunderegg, Photo by Jeff ScovilRice Museum of Rocks and Minerals
How do Thundereggs Form?
The prevailing theory is that water droplets flash to steam and crack open a vug (cavity) in a water-rich, high-silica lava flow. The flow then cools and hardens. Later, the empty vug fills with agate or other minerals.
Why are They Called Thundereggs?
In the 1930’s, excitement grew around unusual round opal and agate filled nodules found in Oregon and other western states.
Thunder Egg Genesis
Inspired by the popularity of thundereggs, a member of the Oregon Mineral Society named J. Lewis Renton submitted an article in 1939 titled "Thunder Egg" Genesis, to The Mineralogist describing these specimens.
A Myth is Born
Renton wrote that the Native Americans of the Warm Springs Reservation in Oregon long ago named them "thundereggs" after angry mountain spirits hurling rocks. However, the historians of the Confederated Tribes of Warm Springs have no record of any such “thunderegg” legend.
