Sapphire is a key item that was first introduced in Pokémon FireRed and LeafGreen. Its sole purpose is to be given to Celio on One Island along with the Ruby. These two items are used to set up the Network Machine so you can trade Pokémon from Pokémon Ruby and Sapphire. In exchange for your efforts in retrieving these items for him, Celio rewards you a Rainbow Pass.
The Sapphire is located in the Rocket Warehouse.