your rock will conform to this. 1/4 inch will do great and bend with ease. Your forms or wall studs also have to conform to this arc.
I have seen this done several ways: One to get 2x6's and scribe your arc on this board and then cut it out with a jig saw. This will become your wall stud for that special area. Caution, the arc must not come within 2 inches of the end of the board. You can even use 2x8's if that will help. They all have to be the same which should not be a problem with a template. Second, they can be build with 2x4's creating a close arc. This method has its downfalls because the wallboard could break if its not cradled properly.
I have also seen prefab bends that you can buy for just this purpose. Do not who carries them but a good contractor will know. hall way arcs are also made pre arc'd. This stuff is like fibreglass or something. Check it out.