It's possible you're thinking of:
* The Diamond and Pearl Versions: These are separate games in the Pokemon series, not items you can find in Platinum.
* The Adamant Orb and Lustrous Orb: These are held items found in the game, but not in Mount Cornet. The Adamant Orb is held by the Legendary Pokemon Dialga and can be found after defeating it. The Lustrous Orb is held by the Legendary Pokemon Palkia and can be found after defeating it.
* The Rare Candy: You can find Rare Candies in the Grand Underground, accessible from Veilstone City. They are useful for leveling up Pokemon quickly.
If you are looking for a specific item, please be more specific about what it is, and I can help you find it!