How to Grow Glittering Crystal Geodes in Eggshells

Part craft project and part science experiment, this hands-on crystal-growing activity will fascinate your kids.

Several colorful eggshell geodes.

Colorful Egg Shell Geodes

Photo by: Debbie Wolfe

Debbie Wolfe

Crystals are fascinating for adults and children alike. While it takes thousands of years for crystal-filled geodes to form in nature, you can grow your own crystals in a day with supplies found at the grocery store. This sparkling eggshell geode is a delightful science experiment that your kids will love.

What You'll Need

  • 3/4 cup plus 2 tablespoons of alum powder (located in the spice section or canning supplies section of your grocery store)
  • egg
  • 2 cups hot water
  • school glue
  • paintbrush
  • food coloring
  • 2 cup glass measuring cup
  • small glasses or pint canning jars

Photo by: Debbie Wolfe

Debbie Wolfe

Crack Egg Lengthwise

First, you'll need to crack the egg. You can crack them in half crosswise like normal if you like. However, to make them look like real geodes, they need to be cracked in half lengthwise. The easiest way to do this is to blow out the egg first, then use a pair of scissors to cut the shell lengthwise.

Glue Alum Powder to Eggshell

After you cut the shell, wash it and wipe it dry with a paper towel. Next, dump about two tablespoons of alum powder into a bowl. Drip some glue into the shell halves and spread it all over the surface of the inside of the egg with the paintbrush. Generously sprinkle the alum powder on the wet glue. (If you want the crystals to grow on the edges of the shells or slightly on the outside, you'll need to glue alum powder in those areas as well.) Turn the shell halves over and gently tap out any excess alum. Place them on a paper towel to dry overnight. Don't skip this step — the alum powder needs to be completely dry to provide a surface for the crystals to adhere.

Photo by: Debbie Wolfe

Debbie Wolfe

Dissolve More Alum Powder in Water

The next day, bring two cups of water almost to a boil and pour it into a large measuring cup. Add the 3/4 cup of alum powder and stir until it's mostly dissolved. You may have a bit of sediment at the bottom of the cup. Divide the mixture into two small glasses or half-pint canning jars.

Photo by: Debbie Wolfe

Debbie Wolfe

Add Food Coloring

Next, add 20 to 30 drops of food coloring to each jar and mix. Let the mixtures cool for 30 minutes.

Photo by: Debbie Wolfe

Debbie Wolfe

Soak Eggshells in Alum Solution

Finally, place one shell half into each jar alum-side up. Gently push the shells to the bottom of the solution with the spoon. Allow them to sit there, undisturbed, for 12 to 15 hours.

Photo by: Debbie Wolfe

Debbie Wolfe

Remove Shells From Solution

After 12 to 15 hours, your alum crystals will have grown! Carefully remove the shells and place them on a paper towel to dry and finish the geode-creation process. If you want bigger crystals, let the shells sit in the solution longer.

A purple eggshell geode.

Eggshell Geode

Photo by: Debbie Wolfe

Debbie Wolfe

Now for the science behind the alum geode-growing process: The geode is formed through a process called sedimentation. Adding the alum powder to the hot water created a supersaturated solution. This simply means that there are suspended particles of alum powder in it, and as the solution cools, these particles of alum begin falling to the bottom. As the alum particles settle on the bottom, they begin crystallizing. Other substances like Epsom salt, sugar and borax also crystallize, but I've found that alum powder makes the best crystals.

Hands holding an eggshell geode.

Egg Shell Geodes

Photo by: Debbie Wolfe

Debbie Wolfe

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