From Bulbapedia, the community-driven Pokémon encyclopedia.
The Magma Stone (Japanese: かざんのおきいし Volcano Stone) is an item introduced in the Generation IV games.
The Magma Stone is found at Stark Mountain. In Platinum, it is explained that the Magma Stone is able to control Heatran, the legendary Pokémon that lives within the mountain. In this nature, it is similar to the Red Chain created by Cyrus.
During normal gameplay, the Magma Stone is not available, and BuckDP or CharonPt takes it. In Diamond and Pearl, Buck returns it after receiving advice from his grandfather, while in Platinum, Charon is arrested and Looker absentmindedly picks it up. Buck puts the Stone back.
Once the Magma Stone has been returned, the player is able to battle and catch Heatran.
If hacked into a game cartridge, the Magma Stone will have no in-game effect.
In Black 2 and White 2, the Magma Stone is obtainable at Route 18. If the Magma Stone is taken to Rebirth Mountain, the player can battle Heatran.