Having destroyed all my terra cotta tiles over the years, I decided to try a slab of slate to cook on. It's an unfinished sample I got at Lowe's yesterday for $10. Currently taking it through temperature cycles to see if it holds up. So far, I can see the layers on the side slightly separating. If it gets to some static state, it should be fine. If it continues to separate, it's back to the terra cotta.