I’m building a new master bath, and would like to use slate on the floor (appropriately sealed) and extend it into the shower and up the shower walls. One of the tile suppliers warned against using slate vertically where water will hit it because it can “bleed” certain minerals, leaving it streaked and nasty-looking. He said it was not an issue on horizontal surfaces, like the shower floor or bench, or where it would only get steam and condensation. Is this a valid worry? The last thing I want to do is chisel it all out in three years, but it is the material I would like to use, if it will work. My builder says the stone should be fine if it is sealed on the edges as well as the face. Who is right?