- This article is about a type of rock; for the rock and roll band, see Thunderegg (band)
Appearance and composition 
Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across. They usually contain centres of chalcedony in the form of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and selenite crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colors. 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.
Thunderegg is not 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 other hand is a specific geological structure. A thunderegg may 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 many 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 especially, remains one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other countries known for their thundereggs include some places in Africa, Poland, Romania, Turkey, Mexico, Argentina, Canada, Australia and France.
According to Pacific Northwest native legend, the Thunder Spirits lived at the highest reaches of Mount Hood and Mount Jefferson, and when they became angry at each other, they threw these spherical rocks at each other.
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 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Oregon.
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- "Oregon Symbols". SHG Resources. Retrieved 2007-09-17.
- Chapter 186 — State Emblems; State Boundary 2005 Oregon Revised Statutes
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- Christie, Tim. “Rock hounds check out goods at 18th annual Gem Faire”, The Register Guard, March 26, 2007, Business and Financial News.