It is surprising to learn of a thunderegg source in the Precambrian rhyolite area of Wisconsin. Most thundereggs found there are hollow lined with chalcedony while others are filled with green to purple fluorite which is quite a rare occurrance among the thundereggs. Only other location is Black Queen of New Mexico which I found fluorite lining a thin cavity within a thunderegg. Many are fractured due to freezing and thawing processes.