Let's clarify what you might be thinking of:
* Fossil Pokémon: Pearl features three fossils, each bringing back a different Pokémon when revived at the Oreburgh Mine:
* Dome Fossil: Revives Cranidos
* Helix Fossil: Revives Shieldon
* Claw Fossil: Revives Anorith
* Amber: While not called "Old Amber," the Amber item is available in the game. It revives the Pokémon Aerodactyl.
Finding these items:
* Dome Fossil: Found in the Route 205 area, south of Oreburgh City.
* Helix Fossil: Found in the Route 210 area, east of Oreburgh City.
* Claw Fossil: Found in the Route 211 area, south of Oreburgh City.
* Amber: Found in the Valley Windworks in the post-game.
If you are thinking of something else, please provide more information about the items you are looking for, and I will do my best to help!