Pearls are biogenic, meaning they are formed by living organisms. They are not sedimentary, igneous, or metamorphic rocks.

Here's why:

* Sedimentary rocks are formed from the accumulation and cementation of sediments.

* Igneous rocks are formed from the cooling and solidification of magma or lava.

* Metamorphic rocks are formed from the transformation of existing rocks under heat and pressure.

Pearls, on the other hand, are created by mollusks, specifically oysters and other bivalves. When a foreign object, like a grain of sand, gets inside the oyster, the mollusk secretes layers of nacre (calcium carbonate) around it to protect itself. This layered nacre eventually forms the pearl.