thumb|250px|A thunderegg from the [[Black Rock Desert in Nevada that has been cut in half]]
A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers.
Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter (one half inch) to over a meter (three feet) across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, Poland, Romania, Turkey, Mexico, Argentina, Canada, Mount Hay and Tamborine Mountain (Australia), and the Esterel massif (France).
Formation
Thundereggs are found in flows of rhyolite lava. They form in the lava from the action of water percolating through the porous rock carrying silica in solution. The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through.
State rock designation
After an initiative by the Springfield Thunderegg Rock Club, on March 29, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly. While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties. The world's largest thunderegg, a 1.75 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon.
Legend
Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson. Thunder Spirits on the mountains hurled the "eggs" at each other.