How to Grow a Geode

Simulate the hollow interior of a rock cavity by forming several sheets of tin foil into a half-egg shape. Mix 2/3-cups each plaster of Paris and Epsom salt with 1/4-cup water. Pour the mixture into the simulated cavity/mold.

Spread the mixture up the side of the mold, allowing the mixture to thicken and stick to the cavity's sides. Let the plaster mixture set up for at least half an hour.

Place the mold into the bottom half of an empty, clean milk or juice carton, open side up.

Mix 3 cups of boiling water with 2 cups Epsom salt and stir until all the salt is dissolved. Let the mixture sit until it cools down to room temperature.

Pour the cooled mixture into the carton, covering the mold, and place the carton in a place with a temperature warmer than 60 degrees Fahrenheit and where it won't be agitated or disturbed. Let it sit for approximately two weeks.

Examine the submerged mold using a flashlight to illuminate the crystals growing in the mold. Remove the mold from the salt solution when the crystals are the desired size.

Preserve the crystals by coating them with a fine coat of spray varnish.