If you give it to Rhydon to hold and trade it, it will evolve into Rhyperior.
Rhydon
on iron island in platinum
Route 228.
in rout 228
If you put it on certain Pokemon and trade it, it will evolve.
A Protecter evolves Rhydon into Rhyperier.
Route 228
Route 228 is where you can find a protector.
It's near Stark Mountain.
down the left staircase in iron island should be there
Trade your rhydon while it is holding a "protecter".
the sand area close to battle frontier