The topic says it all.  I have a granite piece that would make a lovely baking stone.  But:

-do they treat the surface other than polish it?  

-will it explode or anything in the oven?

If anyone has any experience with it, this would be grand to know about.  This piece has been hanging around for about 5 years, and I just found it again.  It is almost useless for anything else.  Measures 22"× 20", and 2" thick.