but not that thin. What I have seen is about one inch thick. Its more like slabs. I saw that around the leading edges of the stone where its the closest to the heat, the polish did seem to tarnish or blurr a bit. Don't know if this would wipe or wax off or what.
Being that thin, I would worry less about heat and tarnish than I would cracking due to human's use as a seat and such. Seems just too thin for a hearth.