This experiment is a specialized type of crystal growing experiment in which we want the crystals to fill in a cavity in a rock.  In nature this process may take tens of thousands of years, but our experiment should be ready in five to ten days.

Step 1 - Make the cavity in which the crystals may grow:
The shell of the geode is made by mixing 2/3 cup plaster of paris with 2/3 cup epsom salts (magnesium sulfate), or whatever chemical you wish to grow as crystals.  Slowly add 1/4 cup water and stir until the mixture is about the consistency of oatmeal.  Immediately pour the mixture into a mold - a large disposable plastic dish or several sheets of aluminum foil folded to make a dish.  With a plastic spoon, start spreading the mixture up the side of the mold.  At first the mixture will slide down, but as it thickens it can be molded against the shell.  Work quickly at this stage because the plaster hardens rapidly.  Let the shell cure for about ½ hour before using it in any experiments.

Note: A fairly large amount of the crystal growing chemical has been added to the plaster that makes up the shell.  These crystals in the shell act as seed crystals for the crystal growing solution, and they firmly anchor the crystals to the rock.

Step 2 - Make the crystal growing solution:
Mix approximately 2 cups epsom salts with 3 cups of boiling water.  Stir until all of the magnesium sulfate (epsom salts) is dissolved.  Let the mixture cool to room temperature.

Step 3 - Make the geode:
A) Place the shell in a convenient container - a half-gallon milk or orange juice container with the top cut off is fine - and slowly pour the cooled solution over the shell.
B) Place the experiment where it will not be bumped, but not on a window, or where the temperature is less than 60 degrees.
C) Each day check to see how the crystals are developing - shine a flashlight through the solution to see the crystals.
D) You may stop the experiment whenever the geode looks like it has enough crystals.  Remove the shell from the solution and let it dry.
E) The crystals that you have grown will dissolve in water, so you may want to protect the crystals by spraying them with varnish.

Note: In humid weather this experiment may take longer than a week to complete.  Other chemicals that may be substituted for epsom salts include sugar, salt, aluminum sulfate, and potassium chloride.

(Contributed by Wayne Mouser, from The Hicksville Gregory Museum in Hicksville, NY via this Internet address:
http:Hnorthport.kl2.ny.us/-npthscc/GREGORY/Growing_Geodes.html)