While thundereggs can be found all over the world, there are a number of beds in Central and Eastern Oregon. On March 29, 1965 thundereggs were declared Oregon’s official state rock!
The prevailing theory is that water droplets flash to steam and crack open a vug (cavity) in a water-rich, high-silica lava flow. The flow then cools and hardens. Later, the empty vug fills with agate or other minerals.