The Magma Stone (Japanese: かざんのおきいしVolcano Stone) is a Key Item introduced in the Generation IV games, and is associated with the Legendary PokémonHeatran. It is a red stone formed when boulders melted in intensely hot magma, then hardened. Magma remains sealed inside.
Route 18 (on one of the cliffs in the northern section)
In Pokémon Diamond, Pearl, and Platinum, the Magma Stone is an item that rests in Stark Mountain. In Pokémon Platinum, it is explained that the Magma Stone is able to control Heatran, the Legendary Pokémon that lives in the mountain. While the Magma Stone exists in the games' data as item, the player cannot obtain it and it has no effect even if hacked into the player's possession.
In Pokémon Diamond and Pearl, Buck takes the Magma Stone from Stark Mountain, but returns it after receiving advice from his grandfather. In Pokémon Platinum, Charon takes the Magma Stone from Stark Mountain, but Looker has his Croagunk confiscate it; after Charon is arrested, Buck takes the Magma Stone back to its resting place. Once the Magma Stone has been returned, the player can encounter a wild Heatran where the Magma Stone originally rested.
The Magma Stone appeared in The Magma Stone. It was removed from its stone altar inside Stark Mountain by Charon, causing Heatran to awaken and go on a rampage inside the mountain. With the help of Looker and his Croagunk, Buck ultimately managed to return the Magma Stone to the altar, resealing Heatran and making it disappear.
In the manga
The Magma Stone in Pokémon Adventures
In the Pokémon Adventures manga
The Magma Stone appeared in the Platinum chapter, where Buck decided to protect it from Team Galactic by taking it away. However, upon removing the Magma Stone, he also unwittingly unsealed Heatran, allowing Charon to capture the Lava Dome Pokémon.