I have a salt mill that is like a pepper mill, except it is obviously filled with salt, a very coarse grade, and then you grind it onto your food. The time has come to re-fill it, and I don't know what type of salt to use. I have a box of rock salt and was considering that, but then it occurred to me that I don't know if rock salt is edible! I mean, it's used to freeze ice cream (ice/salt mixture) but that's the only use I've ever heard of for rock salt. Anyway, does anyone know? Or, what type of salt should I re-fill my salt grinder with and where can I get it? Thanks!