Can you get the adamant orb in Platinum?
Go to Celestic Town and look at the Ruins. (Make sure you have caught/defeated Heatran first and gained the National Pokedex.) Cynthia will come and talk about the Pokemon. You will then be able to catch them at Spear Pillar. Enter the Blue Diamond thing, and you’ll fight Dialga. Leave Spear Pillar, come back, and a pink portal will be there. From there, you will fight Palkia.