Concretion

Concretion is when certain materials get stuck together and harden into a big hard lump. This can happen in things like rocks, where minerals and other things like shells or bones get all mixed together and harden into a big rock with different shapes and patterns. Think of it like making a big ball out of Play-Doh, but instead of Play-Doh it's rocks and shells and stuff. It's like a puzzle all mixed together and then it gets stuck together and won't come apart. Sometimes these concretions can even have cool things inside, like fossils or crystals!