I tried that several years ago on a cold night. You have to have a spot to place the very hot stone(s) or it will burn or melt your bag or tent. The stones I used were scorched from the fire.
I'd be willing to go find the rocks I used, that ARE scorched from being heated, and sell them to you! My mom's mom was full blooded Cherokee.
It's the right size and shape for one, but I don't think you could say it was ever used as one. As toecutter says, there are a lot of stones out there. When I was very young, my three sisters and I each had a "bed rock". We would warm it on top of the wood stove in the evening, and wrap it in a small towel before going to bed. Placed by your feet, it warmed things up nicely, until you fell asleep.