One thing about stone though, because it's not porous the water clings to the outside, and in my experience the gaps between the stones are more easily blocked with water than with something slightly porous like Styrofoam.
I stopped using stones (plural) for that reason as I got more problems with slow drying. However I think one big stone could be a lot better as it reduces the volumne in the pot, adds weight and does not have the same gaps for water to cling in