This is of my 29 gallon. It's the most 'natural' african tank I have. (READ: natural as in lots of rocks) Sorry for the poor image quality.
This is my 45 gallon. Lots of plants and rocks. Probably not really what you are looking for, but just goes to show that not all african tanks have to be nothing but rocks.
Large, rounder rocks will be the norm in the wild. C-Man has a good thing going with what he has, and in the second tank of mine I've used small rounder rocks. It's really up to you as how you want to decorate your tank. There are lots of clifs and caves in the lakes as well.