In the region of Del Norte in Colorado Thundereggs of enormous size do occur: up to 1 m diameter and a weight of 180 kg. However the agate portion is rarely higher in these lithophysae than 3%. Therefore the collectors are breaking the center of the Thundereggs off from the surrounding not agate-bearing remainder. Beside bluish gray agate a very demanded but rare plumeagate is occuring.
Eldon Hunewell describes own experiences on www.peaktopeak.com searching for Del Norte Thundereggs.