Geological Structures Known as Thundereggs
What is a Thunderegg?
A thunderegg is a sphere-like geological structure that 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.
Our thunderegg picture is courtesy of Captain Tenneal. The photo is clickable and redirects to the original image.