A tree falls and is covered in mud over time the wood rite away and minerals seep into were the wood once was the tree now seems like it is made of rock

Petrified

Explanation:

petrified fossils formed when trees that died millions of years ago fell and became covered in mud and sand. Minerals seeped into the wood of the trees. Over time, the wood rotted away and the minerals took its place.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.