This is a cap from a large thunder egg I dug at the Polka Dot mine in Central Oregon. As I was grinding the shell down on this whole egg to make a freeform art piece this section of the core came free. It is the only section in the thunder egg that contains the unique formations you can see inside that are surrounded by a layer of chalcedony. The rhyolite shell has been mostly ground away and is rounded on the back but not polished so as to not damage the inside.
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