The subject or real rocks has come up. I use many and will use many more. My pictures are not as good as some of you people, but you may get the idea. I have found the rocks in many places and it is fun using them. Most are imbedded in foam or plaster or plaster cloth or all three.
Here are two versions of volcanic used as outcroppings, one with a crystal cave


Here is a sedimatary rock

Here is petrified wood used as sandstone. This is still on a diorama


Here are some interesting sedimentary rocks set in plaster and plaster cloth

The point is that many clever things can be done that will be different than carved or molds if not better. Thing about nature is the variety.