I use sand stone, slate, or any flat rock I find out in the world. I scrub, soak, bleach, and sun any new rock before use. By having it under the basking bulb, we simulate the sun-warmed ground outside in the world, and giver them a SAFE source of belly heat. Be sure your basking bulb height is set correctly.@Tom Merry Christmas and thanks for sharing your wisdom! About the flat rock under the basking bulb- is there a particular type you can recommend? Is this so the tortoise can have a nice little hot spot to rest on? Thank you!