Funny, that, Ben! We have some sort of SW desert stone on our house that the builder's wife loved...TERRIBLE in our nasty NJ climate--the stone literally ROTS and breaks apart. We had our entire fireplace and chimney redone--outside in white brick, inside in black slate and granite, because of leaks.. Only the hearthstones, which have sat outside for 14 years now, and are some sort of sand-colored granite have stood up to the weather, but they are not porous at all. You can also seal stone as well, and should, if you want to use flagstones.