Easy Emerald Geode Crystal Project

Colorful Crystal Geode You Can Grow Overnight

You can make an emerald green crystal geode by by growing ammonium phosphate crystals overnight in a plaster geode.
You can make an emerald green crystal geode by by growing ammonium phosphate crystals overnight in a plaster geode. De Agostini Picture Library / Getty Images

Grow this crystal geode overnight using plaster for the geode and a non-toxic chemical to make simulated emerald crystals.

Emerald Crystal Geode Materials

A geode is a hollow rock that is filled with small crystals. This homemade geode is much like a natural one, except these crystals take hours to form rather than millions of years.

  • monoammonium phosphate (also called ammonium phosphate, sold as a plant fertilizer or for use in dry fire extinguishers) 
  • hot water
  • food coloring
  • plaster of Paris 

Prepare the Geode

Prepare a hollow 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 ammonium phosphate, 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 the Crystals

  1. Pour about a half cup of very hot tap water into a cup.
  2. Stir in ammonium phosphate until it stops dissolving. This occurs when a few crystals start to accumulate at the bottom of the cup.
  3. Add food coloring to color your crystals.
  4. Set your plaster geode inside a cup or bowl. You are aiming for a container that is a size such that the crystal solution will just cover the top of the geode.
  5. Pour the crystal solution into the geode, allowing it to overflow into the surrounding container and eventually cover the geode. Avoid pouring in any undissolved material.
  6. Set the geode in a location where it won't be disturbed. You should see crystal growth overnight.
  7. When you are pleased with the appearance of your geode (overnight up to a few days), remove it from the solution and allow it to dry. You can pour the solution down the drain.
  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.

Tips and Tricks

  • If green isn't your color, you can use any color of food color you like.
  • You can grow geodes using other chemicals, like salt, sugar, or Epsom salts.
  • If you don't have Plaster of Paris or simply don't want to mess with it, you can grow the geode inside a clean egg shell. The egg shell is calcium carbonate, so this geode is much like a natural mineral. If you pour the crystal solution over the egg shell, you'll get crystals both outside and inside the shell. To get crystals only on the inside, fill the shell with the solution.
  • An advanced form of this project is to grow crystals inside a "rock" that you can crack open to see the crystals. This takes a bit more work, but produces a cool effect.You can hollow an egg shell by making a small hole in one end of the shell and using a needle to stir up the egg. Shake out the egg and allow the shell to dry before filling the hole with the crystal solution. You may need to use a needle for this. After filling the egg, make sure the hole is at the top, so it won't get plugged with crystals. Allow a day for the geode to fill. Drain off the solution and you're done! You might wish to allow several days before opening this geode, to make certain the inside is completely dry.