The Adamant Orb is a sphere made of diamond, an exclusive item for Dialga. It boosts its Dragon and Steel type moves, but only for Dialga. It can be sold for 5000P. In Diamond, it can be found behind the location where the player fought Dialga, at the back of the Spear Pillar ruins. In Platinum, it, along with the Lustrous Orb, was found in a small back cave in Mt. Coronet only accessible via waterfalls. In Platinum it can summon the legendary Dialga. In Heart Gold and Soul Silver it can be found at the Sinnoh Ruins. In Pokemon White/Black it can be obtained from the Shadow Triad on Marvelous Bridge after the stories completion.
The move says, If an Active Pokémon has Weakness to metaltype, Dialga's attacks do 20 more damage to that Pokémon (before applying Weakness and Resistance).