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Thundereggs Form in Lava Flows

 natural thunderegg with agate in the centre



Thundereggs | A Geological Structure

A thunderegg is a sphere-like geological structure (not a mineral) 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 very ordinary looking outer rock is broken open it reveals minerals within a self-contained centre.



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