Still on my fantasy journey I bought some resin rocks from eBay. They will look great for the Battle of the Five Armies. These were pricey coming in at £25, but they are good and paint up very well. These are painted with GW granite with brown and green washes.
These have been made by casting slate fragments, It's a good idea and the results are very effective. If I had the time I might even give it a go myself...but that is a pipe dream :).