This is actually a pretty interesting topic.
Although I think @ED3945's rockwork is some of the very best I have ever seen, I disagree that scale matters. Rock surfaces are fractal, meaning they are similar at all scales. There is really no way to tell the scale of a rock photograph without some kind of external reference. This has been widely discussed in the geology literature.
BUT, what DOES matter is intended viewing distance. If you are modeling a close-up view of a rock cut, then getting the details exactly right is important, and molds are one good way to do that. However, if you are modeling an escarpment in an attempt to simulate a distant mountain view (which is a kind of forced perspective), then excessive detail is not only a waste of effort, but it is actually harmful. Modeling details that the eye could not resolve in a real scene can break the illusion of distance.
The following scene uses a combination of real rocks slabs (chosen for their lack of fracture details) and rather crude plaster work:
The level of detail is very low, but this contributes to the illusion of depth.
