Hi T; welcome to Permies!
Yes, a granite counter top will work on top of the mass no problem.
The surface of a mass generally runs around 100 F or so. I suspect granite could take 300-400 F before cracking.
It would not do well over the core.
Are you building a RMH yet ? Or still thinking about it.
I highly recommend getting a copy of the Rocket mass heater builder guide, readily available on Amazon.