Baking stones. Thick = Bad. Thin = Good.
Regarding sobering observations: Having replaced my lovely unglazed quarry tiles (~ 1cm in thickness) with shamot firebricks (~ 3cm in thickness), I was ready to enjoy a bread baked on a very smooth surface which was supposed to give me the oven spring I got used to with the quarry tiles.
The firebrick needed hours to heat up properly and even then the bottom of a test loaf was pale and soggy. No fun. Back to the thin tiles that heat up on the fly to roaring temperatures and back to taking care that the bottom of the bread will not burn. I love it.