Nyssa, Oregon, a small town in the state's southeastern high desert region, is known as the Thunderegg Capital of the World thanks to the high proportion of thundereggs found here. Thundereggs are agage- or quartz-filled rocks that look similar to geodes. These stones are typically about the size of a baseball, but they can be both larger and smaller, with the largest known specimen weighing in at more than 1.5 tons.
On the outside, the thunderegg looks like a simple, humble rock, but if it's split open, these rocks are revealed to be filled with naturally occurring deposits of chalcedony, quartz, agate, gypsum, jasper and even opal, providing an often beautiful surprise for those who find them. Once they're split, thundereggs are often polished so the beautiful stone inclusions inside are easier to see.
While Nyssa claims itself as the Thunderegg Capital of the World, this isn't the only place in Oregon where rock hunters can find thundereggs. In fact, most of the eastern part of the state, including the rural counties of Wheeler, Crook, Jefferson, Wasco and Malheur, provide good opportunities for thunderegg discovery. Thundereggs are so much a part of Oregon that they are designated as the state's official rock.