Welcome to the Heart of Oregon Rockhounding
Nestled in the central region of the state, Crook County, Oregon is world-renowned for its abundance of thundereggs—Oregon’s official state rock. With its rich volcanic history, diverse geology, and numerous accessible digging sites, Crook County has become a mecca for both amateur and professional rockhounds.
Crook County is part of the ancient volcanic province known as the Ochoco Mountains. Millions of years ago, this region experienced extensive volcanic activity, producing layers of rhyolitic ash that later hardened into perlite-rich rock. Within these layers, gas bubbles formed cavities—perfect environments for mineral-laden water to deposit layers of chalcedony, agate, jasper, opal, and quartz. The result? Thundereggs.