Make a casting mold for rocks in a short time.
Needed materials are:

- a gauze bandage
- Latex milk
- a stiff paintbrush
- a suitable stone or a piece of coal

Here´s how to do it;

First, use the paintbrush to clean the stone or piece of coal from loose parts. Then paint it with a thin layer of latex milk in order to avoid air bubbles and to make sure that the milk gets into even the smallest crevice.

Once the latex starts to dry, cover it with the gauze bandage and apply another layer of latex milk. Repeat until you have a somewhat stiff mold. Let it dry over night and you´re done.