What happens after buck steals the magma stone on Pokemon pearl?

You go back to the Survival Area and go to his grandfather's house (the house on the left of the Pokémon Centre) and speak to him, then to Buck. Buck will then leave the house. When you go back to Stark Mountain, and into the room the Magma Stone was in, there will be Heatran in it's place, ready to be caught. Best of luck.