What are Thundereggs?
Thunderegg Geological Structure
A thunderegg is a sphere-like geological structure which forms in lava flows.
The centre eventually splits to form a cavity which over time, fills with either chalcedony, agate, opal, jasper or quartz. When the ordinary looking outer rock is broken open it reveals minerals within a self-contained centre.
The picture of the thunderegg is courtesy of Stan Celestian. The photo is clickable and redirects to the original non-compressed image.