The Adamant Orb is a held item that boosts the Dragon and Steel-type moves of Dialga in Pokemon Brilliant Diamond and Shining Pearl.
Dialga is the mascot of Brilliant Diamond, making the Adamant Orb an exclusive item in that game. Shining Pearl players can still see Dialga when talking to Cynthia's grandmother to add it to the Pokedex.
This Legendary Pokemon will be fought as part of the main story at the Spear Pillar. Catching it or making it faint will end the battle, which will leave the Adamant Orb behind for pickup.
Where to find the Adamant Orb in Pokemon Brilliant Diamond and Shining Pearl
If you bought Shining Pearl, the only way to obtain the Adamant Orb is to trade with another player. They can provide it to you as a held item with any creature they choose to trade.
The Adamant Orb will be available in Brilliant Diamond after the faceoff against Dialga. This Legendary Pokemon will be found at the Spear Pillar, near the very peak of Mt. Coronet.
This comes after defeating Cyrus at Team Galactic Headquarters in Veilstone City. Confronting him again on Mt. Coronet will see him summon the Legendary Pokemon exclusive to your version.
Brilliant Diamond players can take on Dialga. You are advised to catch it to add it to your collection. If you cause it to faint for some reason, you can still get your hands on the Adamant Orb.
You will need to beat Commander Mars and Commander Jupiter in a Double Battle and defeat Cyrus one on one yet again. After that additional encounter with Cyrus, Professor Rowan will direct you to battle Dialga.
Once the battle with Dialga is complete, Pokemon Brilliant Diamond players can move down the stairs just past where it was standing. At the bottom of the stairs is a Poke Ball pickup item, the Adamant Orb.