PKMN.NET :: Dex :: Secret Key
DEX -
ITEMS: Secret Key

In-game description: FireRed/LeafGreen: The key to CINNABAR ISLAND GYM's front door. It is colored red and decorated.
Platinum: A high-tech key that has to be used at a specific location. It emits a special electric signal to open a door.
Price: 0

Technical explanation and other notes
FireRed/LeafGreen: Allows access to the Cinnabar Gym.
Platinum: Allows access to the room where Rotom's form can be changed.
FireRed/LeafGreen image:

This item has a specified location in the following games: