Repurposed granite countertop for bench top?

Would a repurposed granite countertop make a decent bench top for a RMH system or would it likely crack from thermal shock?

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.  

And here are a couple of other rmh sites.  https://donkey32.proboards.com/   and http://batchrocket.eu/en/     Both have great information!

A lot of counter tops are fake stone,  glued together composit stone or they are stone but they have waxes,  sealants or other treatments that might  undergo catastrophic change in the face of high heat.