Had good luck with ceiling tile rocks.
Sedimentary:
Have a web article at:
http://www.pacificcoastairlinerr.com/more_rocks/
Something I experimented but never got around to using, cork:
Web article at:
http://www.pacificcoastairlinerr.com/thin_rocks/
Experimented with ceiling tiles as igneous rocks as a volcanic intrusion:
Web article at:
http://www.pacificcoastairlinerr.com/volcanic_rocks/
Thank you if you visit
Harold
Break up some styrofoam to the size you require. The rugged breaks are impossible to duplicate by hand and look great.
Ron from down under.
The OP asked about WS Putty and no one had any answers. Well neither do I[:)] but 20 years ago I made my own molds with medical 4x4 gauze and latex rubber. It was cost effective then and it is now. You just need some local rocks of appropriate texture.
The shorter answer is all of the above. I’ve used most of them at one time or another and they all yield good results. I’m going to use broken ceiling tiles on my next project and it looks fairly easy to do. Forming the rocks facings is actually the easy part. Painting, weathering, and installing them in a realistic manner is the key to a good scene.