So continuing with foam and plaster miniatures, here are some 'natural' looking things you can make pretty simply.
Like always, start with a pink insulation foam base. Make the foam base about the size you want the final piece to be, as the plaster layer on top will not be very thick. I recommend doing this on newspaper because the plaster will adhere to whatever you have the foam sitting on.
Mix the plaster and layer it on. First make sure that wherever you want plaster to be, the plaster is at least a few centimeters thick so you will be able to sculpt it. As the plaster is drying, start carving. One really easy way to make rock-like features is to crinkle a ball of aluminum foil, and press it into the plaster. Toothpicks can also be used effectively to draw strirations into the rocks to make them look layered.
If you draw the lines in with a toothpick, gently press the aluminum foil in afterwards to hide the lines a little. One of the key factors to all this is to make the impressions when the plaster is dry enough to hold the shape, but wet enough to be moldable. This may take some practice to get used to. Once the plaster dries a little further, you can scrape off unwanted bits or carve stronger lines into the plaster.
For painting these rocks, I used the same idea as I did with the Argonath, and started with black and added succesively whiter layers of paint untill I was just drybrushing white onto the surface. The ground is just brown with green fake terrain grass glued on.
Bark chip Rocks:
One way to make small versions of big rocks is to use barkchips or woodchips. These have naturally fine layers of detail, which, when painted, do a good rock impression.
To make them, first dry out bark or wood chips (I use bark). Paint black. It can be a struggle to get paint down into the finer crevices, but the more thorough a job you do here, the better the final product will be. Now add a layer of dark gray paint, and don't paint all the way into the deepest crevices. Next, a layer of lighter gray, again not painting as deep as the previous layer. Continue this, and as you get lighter, use less paint on your brush each time to help keep the paint from dripping down. Eventually, drybrush a layer of almost white to finish. The samples I have here aren't great paint jobs, but show how different textures of bark can make different looking rocks.