There are 2 ways to create realistic rocks: toilet paper and sylicon mould.
Toilet paper: on the papier-mâché surface, where the rock would set, spread with a brush vinyl glue. Apply toilet paper, blutting with a brush to make it better bond to the structure below.
When the glue is dried, the paper has irregularity like the rock stratification. Use light colors (gray or ochre with their nuances) to make your rock realistic.
Syliconic mould: first search some little rocks in Nature. At home clean them to remove dust and earth. With syliconic rubber make a mould spreading on the rock. It could be necessary to renforce the mould with some gauzes. After drying divide the rock from the mould. To create a copy of your use plaster. Drain it on the mould. After remove it softly. Paint it with graor ochre. Then fix the mould on your model.