Phenocryst

A phenocryst is like a special rock hiding inside another rock. Imagine if you had a cookie dough, but you put a special candy inside before you baked it. That candy is like a phenocryst.

The bigger main rock is called a groundmass and inside it, there is another smaller rock that grew differently from the rest. It's shaped differently and looks different. It's like a special guest arrived at a party dressed up in a really fancy outfit.

Scientists study phenocrysts because they can tell us a lot about how the main rock was formed. And since the phenocryst grew differently, it can have different minerals that give us clues about how hot or cold the rock was, or how fast it formed.