Most of us have left-over plaster at one point and most of the time it is discarded. There is really no need to through it out.

Once your plaster is dry, place it into a plastic tub or bowl and break it apart with a hammer. What you are left with is a pile of rocks and boulders that can be painted to match your scenery. Further more, rubbing a handful of them together lightly will smooth them out a bit which can look like river rock.
A really simple and easy trick to add more texture to your scenery.
If you would like to read further, Ed Traxler has a great article on creating layered rocks formations using crushed plaster.