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thunder egg
It's what is commonly known as a thunder egg.
They are usually formed within silica rich volcanic rocks such as rhyolite lavas as a sort of concretion. As the deposited lava cools trapped gases in bubbles or vesicles gradually solidify as the pressure and temperature conditions change during cooling of the lava.
Sometimes they can have very coloiurful crystalline minerals in their cores called spherulites.
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