Rocks with a surprise inside

Geodes are rocks that contain crystals within them. On the outside they look like ordinary rocks, but on the inside they are very beautiful! In nature, geodes take a long time to form, but you can create a faux geode in just a few days. Through this experiment, you will learn a little more about the crystallization process.

For this experiment you will need:

✔ 500 grams of potassium alum powder (to make about 6 geodes)
✔ Real eggshells or halved plastic eggs
✔ Brush to apply the glue
✔ Egg crate to dry and store the geodes
✔ A spoon
✔ Container for mixing the solution
✔ A cup to measure the powder and liquid
✔ liquid food coloring (optional)


For this project, ask an adult to save the eggshells the next time you use eggs for breakfast or in a recipe.

Clean the egg shells making sure to remove all the membrane from inside the egg, if any membrane remains it is possible for mold to grow inside the shell. Using a paintbrush, apply a thin layer of glue to the inside and edges of the shell. Before the glue dries, dust the shell with potassium alum powder. You must wait until the next day for the glue to dry and harden before adding the potassium alum solution.

⛔ Ask an adult to pour a cup of almost boiling water into a container. Next, add about ½ cup of the potassium alum powder and mix with a spoon until the powder has dissolved. Keep adding small amounts of the powder until the solution is supersaturated. Supersaturated means that the water has absorbed as much as it can absorb and any solids you add will not dissolve. If you want to add food coloring, use 30 to 40 drops. Let the solution cool for at least 30 minutes.

Carefully pour the solution into the eggshells, filling them completely. Let the shells sit in the egg carton overnight. You will notice that crystals will form inside the eggshells as the water evaporates. The more days, the more your crystals will grow. Thanks to science and patience you will have created a beautiful imitation geode.


The alum solution contains suspended particles of alum powder and as the solution cools these alum particles fall to the bottom. When the alum particles settle to the bottom, they begin to crystallize. The alum powder coated eggshell gives the suspended alum particles a surface to which they can easily adhere. In nature, geodes form in a similar way; the mineralized water seeps into the air pockets of the rock forming crystals.