For stubborn water mineral deposit stains, I use a flexible black mesh made by 3M you can pick up at any hardware store. Rub it over the stain and it should come off. I use the mesh primarily in the toilet for that 'water line' stain.
Pumice scratches the enamel and exposes the porous material underneath, which stains even faster.
This 3M mesh does not scratch. Joseph Try using Bar Keeper's Friend. It is awesome for getting out difficult stains. Trisha I use lemon juice for cleaning those rust rings out of my tub. I do recommend fresh juice from the lemon because I don't trust the fake stuff on the shelves.