Non-toxic and looks/feels like stone.
Can be tinted any color with non-toxic pigments.
I've considered it myself.
They'd probably 'hold heat' just as well as the clay flower pot hides some keepers use.
I was worried it would 'dissolve' if it got wet but I had a box of it accidentally get exposed to air and no matter how long I soaked it, I could not 'resurrect it' to usable, pliable clay again.
It gets really hard.
You can also bake it and seal with non-toxic acrylic sealer.
You also could look for a potter whom offers classes and see if you could spend an afternoon building clay hides and have them fired and glazed. I know one who you buy the clay amount from her and then dry is according to the sheet she hands out and then return it for a fee she then fires is for you.
Yes, I thought of this because my mom actually makes sculptures at a studio with a kiln! I starts again ant the beginning of the school ear, I'd have to wait until then..