Geodes are rocks with air pockets inside that have crystals growing inside them. They can look very ordinary on the outside, but beautiful on the inside! In nature, geodes take a long time to form, but you can make a model geode in just a few days.

What You Need:

(Note: You can also make an Egg Crystal Geode with an eggshell as your rock, instead of making a plaster one.)

What You Do:

Grow some seed crystals:

  1. Mix 1/4 cup very hot tap water with as much alum as you can dissolve in it. (Keep adding alum until it stops dissolving and you see alum particles sinking toward the bottom.)
  2. Pour some of the solution into a shallow dish, like a saucer, being careful not to pour any of the undissolved material from the bottom. (You can pour through a coffee filter.)
  3. Set the dish aside overnight, or longer if you wish. The next day you should see small crystals growing in the dish. Pour off the remaining solution and scrape the crystals off.

Make the plaster “rock”:

  1. Find a mold for your plaster geode. You can use one of the cavities in an egg carton, make a dish out of aluminum foil, or line a small bowl or cup with plastic wrap.
  2. Stir some water and plaster together until it’s about the consistency of oatmeal. Add the seed crystals you grew.
  3. Spread the plaster into your mold to create a small bowl shape. Let it set for about half an hour, then remove it from the mold and let it dry another half hour or until it is completely dry.

Mix the solution:

  1. Mix 1/2 cup very hot tap water with as much alum as you can dissolve in it, just as you did to grow your seed crystals.
  2. Add some food coloring if you desire. The crystal structure won’t incorporate the color, but the plaster will be colored behind the crystals, which will make it look like they are colored.
  3. When the solution is saturated, set your geode in a small bowl or container (something that won’t get stained by the food coloring) and slowly pour the solution over it until it is just covered. If your container is too big, the solution might not cover the whole geode, so try to pick something small but deep enough.
  4. Set it in a safe place for a few days and let the crystals grow! Whenever you think your crystals have grown enough, remove the “geode” from the solution and let it dry.

What Happened:

The seed crystals in the saucer grew because alum molecules in the solution met up with each other and joined together in a crystal structure. Adding them to the plaster provided sites to encourage crystal growth in the second solution: the alum molecules were able to attach onto crystal structures already present and embedded in the plaster, which makes them grow faster and stronger and look more like a real geode.