The mine was originally opened to mine the cinnabar deposits and the thundereggs were considered a nuisance before it became a fee-dig site. When the mine was first opened, several beds of thundereggs with fully silicified hard shells have been found, but as the mining progresses into lower levels (and other beds), those with "rotten" and porous unsilicified shells are becoming more common, hence, "New Buchanan thundereggs". Some thundereggs found there contain reddish cinnabar inclusions while others contain either messy or neat horizontally banded agates and it's tricky to tell which way to cut since they are quite rounded without any ridges showing. The "messy" look resulted from cracking and flaking of each silica when it dried after being deposited (like mud). The site is now closed to collecting.