Depending on the minerals present in the area, the inner core of thunder eggs can vary widely from region to region and even from site to site. Different temperatures and fluid compositions account for the differences in color and texture within any given thunder egg.
Thunder eggs are formed only in flows of rhyolite lava. As the lava cools, a hollow of gas is trapped in the rock. Over thousands of years, small fractures in the rock allow for the deposits of mineral laced waters to seep into this hollow space. These mineral deposits continue to collect within the hollow void over years. Eventually, a center core of chalcedony is formed, which can also be laced with other minerals like as jasper and agate.
Thunder Eggs have the appearance of a very plain looking rough rock on the outside, a careful slice and polish reveals a beautiful hidden core.