Amber
From Bulbapedia, the community-driven Pokémon encyclopedia.
The amber is an item you can obtain in Kanto (except during Pokémon Gold, Silver and Crystal. Taking it to the Lab on Cinnibar 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.
