I thought I might share the way I "make" rocks.

In our garden centers you can buy bags of pieces of bark, normally used for the decoration of your garden. However, as you can see on the pictures below, these pieces make excellent rocks, you don't even have to paint them, although I do give them a wash and do some dry-brushing on them. For background I use oasis, a product used by florists, which is very light, cheap and it can easily be cut into shapes and glued together or to you layout with white glue. I hope the attached photo's give you an idea about how I used these materials. 

 (Moderator edit: made the linked photos into inline images - full images are 2000x1500)