The most famous thunderegg dig in the country, a working ranch in central Oregon that has let rockhounds pick its beds for generations. You drive out to the beds, dig thundereggs, and pay by the pound at the shop, where they will saw them open so you can see the agate and moss inside on the spot. Oregon made the thunderegg its state rock on the strength of ground like this. PUBLIC fee dig, near Madras (Jefferson County).
Thundereggs (Oregon state rock)
See it on the map
Other places on the map within a couple of hours.