A good heat absorption tile that is slightly rough. Slate is recommended but any tile that will absorb heat really well and isn't slick(when you run your finger pad over it, does it feel rough?) works as well.
I went with the natural slate tile that is used mostly for outdoors. It has natural texture and unglazed so traction is not an issue. it is thicker than standard tiles though, so be prepared for that as well as extra weight.
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!