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Strange potato rock with crystal inclusions

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That is a septarian nodule that has partly eroded. It’s a type of concretion. They start off as a ball of mud, and dry out, which leads to internal cavities called septs. As water brings minerals inside, (usually calcium carbonate) they form crystals (typically aragonite and/or calcite) inside. They are quite stunning when cut an polished. 

Also, the mud balls usually form around something, called a nucleus. Sometimes it’s just a grain of sand, and sometimes it’s an interesting fossil. They can be found all over the world.

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