Amber
From Bulbapedia, the community-driven Pokémon encyclopedia.
The amber is an item you can obtain in Kanto (not including Pokémon Gold, Silver or Crystal). Taking it to the Pokémon Lab on Cinnabar Island will give you an Aerodactyl.
The player is given the amber by a scientist in Pewter City Museum. You will need to use Cut to enter a door on the eastern side, which leads to behind the counter you normally see when you enter the main doors.
In the anime Gary Oak obtains an amber and uses it to revive an Aerodactyl in Putting the Air Back in Aerodactyl
