You don't get the magna stone from buck in Pokemon platinum.
Go to Stark Mountain and get the Magmar Stone after Buck has put it back.
Buck lives in the Survival Area in Pokemon Platinum. Buck is part of a quest involving Stark Mountain and Heatran.
Buck is a boy
After buck has gone back into Stark mountain with it you go back in talk to him he leaves you take the magma stone and Heatran apppears (lvl 50).
You can catch Heatran in stark mountain. You end up defeating team galactic and you lead buck to the magma stone. Heatran will appear after you speak with buck in the back of stark mountain. I think your thinking of Diamond or Pearl, I tried it in Platinum. Doesn't work.
you go to the resort area
You don't need a stone after Charon is arrested in the room (somewhere in stark mountain) go and talk to Buck's grandfather and Buck in their hotel and than go again in stark mountain in the room where Charon was arrested, u will see Heatran their ( but heatran is a useless pokemon, though legendary (according to me) )
He is outside and (later) inside Stark Mountain.
i believe buck has one in the battleground
in the battlegruond buck has Shuckle Umbreon Dusknoir Torkoal Claydol
at stark mountain. you have to beat team galactic and go thur stark mountain again. (buck, the guy that follows you, will put the stone back after you get it) YOU MUST SEE ALL 210 POKEMON BEFORE YOU GO TO STARK MOUNTAIN (HEATRAN IS THERE)
Shuckle, Dusknoir, Torkoal, Umbreon, and Claydol.