How To Make a Crystal Geode

Close-up of Amethyst Geode
Adrienne Bresnahan / Getty Images

Natural geodes are hollow rock formations that contain deposits of crystals. Assuming you don't have a geological timeframe to obtain a geode and don't want to buy a geode kit, it's easy to make your own crystal geode using alum, food coloring, and either plaster of Paris or an eggshell.

Crystal Geode Materials

  • Alum (found with spices in the grocery store)
  • Hot water
  • Food coloring (optional)
  • Plaster of Paris (found in hobby shops) or an eggshell

Prepare the Geode

There are a couple of ways you can go here. You can crack open an egg and use the rinsed shell as a base for your geode or you can prepare a plaster of Paris rock:

  1. First, you need a rounded shape in which you can mold your hollow rock. The bottom of one of the depressions in a foam egg carton works great. Another option is to set a piece of plastic wrap inside of a coffee cup or paper cup.
  2. Mix a small amount of water in with some plaster of Paris to make a thick paste. If you happen to have a couple of seed crystals of alum, you can stir them into the plaster mixture. Seed crystals can be used to provide nucleation sites for the crystals, which can produce a more natural-looking geode.
  3. Press the plaster of Paris against the sides and bottom of the depression to make a bowl shape. Use plastic wrap if the container is rigid, so that it's easier to remove the plaster.
  4. Allow about 30 minutes for the plaster to set up, then remove it from the mold and set it aside to finish drying. If you used plastic wrap, peel it off after you pull the plaster geode out of the container.

Grow Crystals

  1. Pour about a half cup of hot tap water into a cup.
  2. Stir in alum until it stops dissolving. This occurs when a little alum powder starts to accumulate at the bottom of the cup.
  3. Add food coloring, if desired. Food coloring does not color the crystals, but it does color the eggshell or plaster, which causes the crystals to appear colored.
  4. Set your eggshell or plaster geode inside a cup or bowl. You are aiming for a container that is a size such that the alum solution will just cover the top of the geode.
  5. Pour the alum solution into the geode, allowing it to overflow into the surrounding container and eventually cover the geode. Avoid pouring in any undissolved alum.
  6. Set the geode in a location where it won't be disturbed. Allow a few days for the crystals to grow.
  7. When you are pleased with the appearance of your geode, remove it from the solution and allow it to dry. You can pour the solution down the drain. The alum is essentially a pickling spice, so while it isn't exactly good for you to eat, it isn't toxic either.
  8. Keep your geode beautiful by protecting it from high humidity and dust. You can store it wrapped in a paper towel or tissue paper or inside of a display case.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "How To Make a Crystal Geode." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). How To Make a Crystal Geode. Retrieved from Helmenstine, Anne Marie, Ph.D. "How To Make a Crystal Geode." ThoughtCo. (accessed June 5, 2023).