Don, I'm going to have to echo Gene's admonishment. The only thing you can dig out of the ground and successfully expose to extreme temperature changes is soapstone, and that's only a particular type of soapstone. I think the odds of granite exploding aren't huge, but there is a risk. The risk is even greater, though, that the granite will crack. Why use a stone that you know will fail, sooner rather than later?
I guess, at some point, baking stones were really 'stones,' so it kind of make sense for the name to stick around, but, these days, real stones should never be used for baking- the composition and internal structure vary WAY too much to predict baking behavior.