A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that's formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a metre across. They usually contain centres of chalcedony which might have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various additional mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them might reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like additional agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.
Thunderegg isn't synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the additional hand is a specific geological structure. A thunderegg might be referred to as a geode if it has a hollow in it (see illustration of Gehlberg specimen), but not all geodes are thundereggs because there are a large number of different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.
Thundereggs are found globally wherever conditions are right. In the USA, Oregon remains one of the most famous thunderegg locations. Germany is additionally an important centre for thunderegg agates (especially sites like St Egidien and Gehlberg). Other countries known for their thundereggs include a few places in Africa, Poland, Romania, Turkey, Mexico, Argentina, Canada, Australia and France.
Thundereggs are found in flows of rhyolite lava. They form in gas pockets in the lava, which act as molds, 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 colours come from differences in the minerals found in the soil and rock that the water has moved through.
State rock designation
On March 30, 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 tonne specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Oregon.
Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson (Oregon). Thunder Spirits on the mountains hurled the "eggs" at each other.