I call this a double yolk thunderegg because two of these round rocks formed together. Thundereggs are Oregon’s state rock. You never know what’s inside until they’re cut open, like this one, or already cracked open.
This double yolk thunderegg is from Richardson’s Rock Ranch, north of Madras, Oregon. Though you can no longer dig there, you can purchase cut, polished, and raw rocks of many kinds from their large store.
If you want to dig for thundereggs, visit Priday Polka-Dot Agate Beds, about 9 miles northeast of Richardson’s. See my post about this amazing place if you want to uncover ENORMOUS thundereggs.