Jeffrey Anderson is describing this location on his website dwarve´s earth treasures:

“...There is a rhyolite area in the Big Bend of Texas where the thundereggs are found, but the quality of those thundereggs are so poor that they had been ignored. Most thundereggs found there are fractured and filled with macrocrystalline quartz and hydrothermal chalcedony. Some specimens are found to contain colorless to ivory colored banded agates with lots of hoziontal opal layers..”