To quickly build a rock that looks great, we need a cardboard box, newsprint, white paper towels, coffee, adhesive tape, glue, white glue and cardboard.
We made balls with newsprint and we stick it with adhesive tape in the cardboard box. It will be firmly fixed.
Then you wrap the box with the paper balls with kitchen paper.
Then you make a mixture of equal parts of white glue and coffee and you must paint the kitchen towels. If you break the paper put another piece of paper and repaint.
 Let it dry a couple of hours and will stay hard as a rock.
If you have time you can paint the rock with gray crayon.