Fossil

A fossil is an item that can be found in certain caves and dug up in the Underground. Fossils can then be resurrected into the Pokémon it is a fossil of. Each Generation has its own set of fossils consisting of at least two stage-two evolutionary families with Generation II being the exception.

Resurrecting
In order to resurrect the fossil into a Pokémon, you'll need to visit a building, usually a museum.
 * In Pokémon Red, Blue, Yellow, FireRed and LeafGreen, and Let's Go, Pikachu! and Let's Go, Eevee!, visit the Pokémon Lab on Cinnabar Island and enter the room on the far right.
 * In Pokémon Ruby, Sapphire, Emerald, Omega Ruby and Alpha Sapphire, visit the Devon Corporation in Rustboro City and have it resurrected by the scientist.
 * In Pokémon Diamond, Pearl, and Platinum, visit the Oreburgh Museum in Oreburgh City.
 * In Pokémon HeartGold and SoulSilver, visit the Pewter Museum in Pewter City.
 * In Pokémon Black and White, visit the Nacrene Museum in Nacrene City.
 * In Pokémon X and Y, visit the Fossil Lab in Ambrette Town.

Trivia

 * Generation I is the only Generation to have more than two fossils, Old Amber.
 * Generation II, VII and VIII are the only Generations without fossils.