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Emerald Crystal Geode

Easy Crystal Geode Project

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This is a plaster geode of emerald ammonium phosphate crystals.

This crystal geode was made by growing green-tinted ammonium phosphate crystals overnight in a plaster geode.

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

Crystal Geode Materials

  • 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 (found in hobby shops)
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 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 distubed. 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.

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