A Thunder Egg “Thunderegg” is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across.

They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions.

Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours.

A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.
Cookie Monster //  This rock is known as a "Thunder egg" and the outer shell is comprised of rhyolite. Credit:  "Bill the Egg Man"

Amazing unique bird-like agate. This rock is known as a "Thunder egg" and the outer shell is comprised of rhyolite. From Chihuahua, Mexico. Credit: Captain Tenneal

A beautiful Laguna Agate from Mexico with a whimsical pattern that looks like the face of a laughing clown. Credit: Uwe Reier
A thunder egg rock and finding Friendly Ghost,Caspar is one of the best known rocks at the Crater Rock Museum, United States.

A woman presents a hooded owl agate mineral from Mexico at the 42nd Mineral Show and Geological Trade Fair in Munich, Germany. Credit Classic rock and gem

Agate - face at Munich mineral show .Credit:  Martin P. Steinbach
An agate slab with a happy smiley face in its brown bands.

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