You can use latex rubber to make molds of rocks you like. Remember to put a layer of gauze between the coats of latex, though. Then, using Hydrocal or plaster, you can make many rocks. I tend to pick large rocks to mold and break up the large castings into smaller ones, and, with a rotation or two, nobody notices the similarities.
Here to help Murph

i use mother natures supplies. Bush parts for trees and mount dried greeb stuff with 3M spray glue and natural rocks from around

Okay fellow modelers. Hold my feet to the fire on this one. I needed a tree for my diorama. It had to be shaped just so to fit and cover what I wanted covered, but not cover what I wanted to show. Therefore, natural roots were out of the question. So, I started the "tree from hell". over 100 hours later, I am almost done and I will post pictures of my steps when I figure how.
It is 12" tall and I didn't want to use the shredded foam or herbs methods. The tree wold be a dominent part of the dio and had to stand scrutiny. Hammer me until I post the article.

Sealhead