The Dollar stores and some craft shops have the polished stones BUT they are clear coated with an acrylic coating. To check them place one in a small container and cover the rock half way up with (real) nail polish remover, lacquer thinner or MEK for a day to see if it dissolves it.
This coating will flake off after being in water for a while and algae eaters (especially pleco's) will eat thru it.
Real polished stones (tumbled is one way) should just be washed with hot water to clean anything they could have picked up in their handling process.
I've run into the acryllic coated stones at home depot and Lowes before. It's faster and cheaper to manufacture them this way. They are usually marked as DECORATIVE stones however...rather than polished. Nothing at all wrong with properly cleaned polished/tumbled stone.
