Thundereggs are not geodes, but they are so similar that they deserve at least one locality mention in this article. The state of Oregon is the most famous thunderegg locality in the world. Thundereggs are found in rhyolite and tuff deposits in many parts of the state. In 1965 the Oregon Legislative Assembly issued a resolution making the thunderegg the official state rock. The state has a thunderegg museum and locations where you can enter, pay a small fee, and look for thundereggs to take home.