| Thunderegg is official Oregon's state rock.
Formed millions of years ago, Thunderegg (Thunder egg) is a round nodule-like rock formation, filled with agate, jasper, opal and other minerals. Thunderegg is also called agate-filled or jasper-filled nodules. The size can vary from an inch and less to over three feet in diameter (1 m). The exterior surface is white-gray to brown, looks rough and unattractive but the inner content may surprise you with its impressive crystal formation and colorful mineral patterns. To reveal the natural beauty of thundereggs, they should be cut in half and polished.
The name was given according to an old legend of the Warm Springs Indians. Two neighboring snow-capped picks - Mount Jefferson and Mount Hood got angry with each other and threw eggs of Thunderbirds which were nestled in both mountains. "Thunder spirits of the mountains who lived in the craters hurled the nodules to the accompaniment of much lightning and thunder".
The oval-shaped thundereggs have been formed in the rhyolite lava flows and tuffs within the gas or steam pockets that served as molds. Typically, the cavity was filled with silica-rich fluids that later solidified and crystallized. Mineral impurities collected along the bands creating concentric colorful rings.
Note. Hollow thunder eggs are known as geodes.
How to Spot Thundereggs?
Though thundereggs appear as rocks, they can be easily identifiable by the abnormally round and bumpy brown-gray exteriors. Usually, thundereggs lie relatively close to the Earth's surface, embedded in the clay of the tuff, where they were formed. During the time, the tuff has been decayed to soft mud and clay, making rock-like thundereggs noticeable.
Where to Find Thundereggs?
The regions of Central and Eastern Oregon are the most popular in the world for thundereggs hunting. The best productive and free areas are located near Prineville in Ochoco National Forest (Central Oregon) and Succor Creek Creek Canyon (Eastern Oregon). Thundereggs also can be collected at pay-to-dig sites northeast of Madras.
Gems and Minerals in Oregon. Oregon Department of Geology and Mineral Industries. OD http://www.oregongeology.org.
Dan R. Lynch, Bob Lynch. Rocks & Minerals of Washington and Oregon: A Field Guide to the Evergreen and Beaver States (Rocks & Minerals Identification Guides). 2012.