try this tutorial .
No need to follow it but at least have a look so that U have an idea how the things work .(since U are still learnng)If U want to add a realism to rocks here is a vid…
Ue4: advanced materials (Ep.1 Using CrazyBump) - YouTube(this is Ur answer hope this is understandable)
One thing I need to apologize is I won’t be too active for a week as I am void of a good internet connection so it might happen that I am not that prompt in replying back to Ur queries.
Another thing U should also know how to create realistic textures and bump maps too using any good photoshop tool
if possible so that U get the best of results. (try the CrazyBump software works for me)