Porous stones or charcoal briquettets
Vinegar
Salt
Warm water
Small enamel or glass bowl

Add 3 tablespoons of salt to a half a bowl of warm water and stir until it dissolves. Keep adding salt until no more will dissolve. Add a tablespoon of vinegar. Fill up the bowl with stone or charcoal. Leave on top of refrigerator for a couple of days. Take it down and notice that crystals have begun to form. As the water continues to evaporate, the bowl will be filled with rock crystals.