.]] This article is about a type of rock; for the rock and roll band, see Thunderegg A thunderegg (or thunder egg) is a geological structure, similar to a geode, that is formed within a rhyolitic lava flow. Thundereggs are not actually rocks, but rather geologic nodules which occur within other igneous rocks. Appearance and composition Thundereggs are rough spheres, most about the size of a baseball. They contain centers of chalcedony in the form of agate, jasper, or opal. Thundereggs look like ordinary rocks on the outside, but slicing them in half may reveal intricate patterns and colors, which make them valuable in jewelry. History 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 oth