To be honest and with respect to the artists that use it, I\'m not very happy with this current cork trend. It was okay the first couple of times, but it\'s starting to be really overused. And the overuse just makes it more obvious that it\'s cork.
I have heard people praise Oak bark as a pseudo-rock.
Slate is easy:
http://www.coolminiornot.com/article/aid/367
I\'m having fun this week making molds of rocks that I\'ve found lying around. The advantage of casting plastic/plaster rocks is that you can easily drill holes in them for pinning... I need a ton of rocks for a current diorama project, and nothing beats casting your own.