THUNDER EGG POLISHED HALF

You are purchasing the thunder egg half pictured. Pictures show front and back. 

approximately 3 3/4" in diameter at its longest point. 

A thunder egg is a nodule-like rock, similar to a filled geode that is formed within rhyolitic volcanic ash layers. They usually contain centers 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. Thunder eggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colors. A characteristic feature of thunder eggs 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.

Thunder egg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunder egg on the other hand is a specific geological structure. A thunder egg may be referred to as a geode if it has a hollow in it, but not all geodes are thunder eggs because there are many different ways for a hollow to form. Similarly, a thunder egg is just one of the forms that agate can assume.