Thunderegg

A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers.

A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

Other places known for thundereggs include Ethiopia,[2] Poland,[3] Romania, Turkey, Mexico, Argentina,[4] Canada, Mount Hay[5] and Tamborine Mountain (Australia),[6] and the Esterel massif (France).

[11][12][13] While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties.

[14] The world's largest thunderegg, a 1.75 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon.