After seeing the "South Pyramid" thundereggs displayed at Paul "GeodeKid" Calburn's collection housed at Las Mimbres Armory Museum in Deming as well as the mysterious thundereggs in the rock piles of several local rock shops, we started searching for the source of those thundereggs. After a few failed attempts (and lot of 4x4 driving), we finally located the pits on the south side of South Pyramid Peak where Paul C. and Pete Ghitney apparently did some digging and the fragments were left about.
    It appears that the thunderegg-bearing strata is quite thin (about a thunderegg thick) in a thick and weathered blank perlite/tuff mixed with black volcanic glass and something tectonic happened that caused so many thundereggs to be fractured (about 95% are fractured). The strata is capped with a thick and hard reddish rhyolite bed making it impossible to dig to follow the strata unless one use dynamites to blow that caprock off. Then there's a risk of damaging the thundereggs from the blast since they're just under the caprock.
    The agates in the thundereggs are generally colorless sometimes with watered-down pink, yellow and red colors and several were found with black, white, and yellow sagenite inclusions. There are also small red to yellow agates filling in the brecciated gaps found within the rhyolite caprock. If you have a sagenite thunderegg from an unknown location in New Mexico, check for biotite bits in the shells.
    We were dismayed to learn that the old diggings are located on a state land surrounded by private ranches so it's best to leave it alone and I wouldn't be surprised if the gates are locked now. But at least, that visit helped us ID those those mysterious thundereggs we obtained from local rock shops.