I would select the rock kind from a site like Poliigon.com, it's with a free trail.
Or maybe defuse in the color and bump it with a noise mapping through a mixshader.
As far as my noob experience goes, that seems logical.
If you have the same kind of stone nearby, make a closeup picture of it with a decent camera, and run it through a program like Crazybump
That'll look fine.
I could try it on the blender file if you want.
Also a nice learning experience for me.