How to make eggshell geode crystals with alum. This easy tutorial will show you how to turn eggshells into colorful geodes in a few simple steps. Kids will love watching the geodes grow in the alum water and marvel at the bright crystals. This chemistry experiment is perfect for a science fair, learning activity, or educational boredom buster.
My younger two children are at such a fun age right now. At 8 and 10 years old, they’re finding interest in science projects we can make at home. At this point, I think we have tested every slime recipe we can think of and are always looking for new science experiments to try.
When my now teenager was little, he used to collect rocks, gems, and arrowheads. Recently, he showed his younger siblings an old geode he’s had for years and the kids were fascinated by the shining crystals inside. This sparked their curiosity in how geode rocks form crystals inside. So, we spent our summer learning about geodes and the incredibly beautiful crystals hidden inside the nondescript rocks.
What are geodes?
Geodes are formed in sedimentary or volcanic rocks. The rocks are usually round and look plain on the outside. Once cracked open, beautiful crystals shine brightly inside the hollow rock. Most geodes contain amethyst or clear quartz, but there’s no way of knowing until the geode is opened.
There are many ways to make homemade geodes as a science experiment for kids. Today I’m going to share how to turn eggshells into colorful geode crystals with alum powder, water, and gel food coloring. This chemistry experiment is easy to make however, it does require some time for the crystals to grow. With a little patience and time, you can turn eggshells into a rainbow assortment of geode crystals.
I’ve added Amazon affiliate links below to the products used to make this project. There’s also a quick video tutorial with step-by-step instructions.
More DIY craft ideas for kids
- How to make a Dinosaur Dig Excavation Kit
- Glow in the dark slime
- Rainbow tie-dye shoes
- Unicorn bath bombs
DIY Geode Crystals Supplies
How to make eggshell geodes with alum
- Begin by poking a hole on one end of the eggshell and removing the egg into a bowl. Carefully cut the eggshells lengthwise and wash with soap. Dry the eggshells on a soft towel.
- Brush a small amount of white glue around the inside of the eggshell. Sprinkle alum powder over the glue. This will give the alum powder in the water something to grow on. Allow the eggshells to dry completely before placing in the water. Tip: prepare this step the night before so they’re ready in the morning.
- Pour 2 cups of boiling water into a mason jar or wide glass.
- Stir in ¾ cup alum powder until it dissolves.
- If desired, add 1-2 drops gel food coloring to the water. Let the water cool for 30 minutes.
- Place the eggshell in the water and let it sit overnight.
- Carefully remove the egg with a spoon and gently place it on a soft towel to dry. The crystals will be very delicate while still wet. As the geode dries, the crystals will create a sturdy bond. It’s best to handle them with care so you can enjoy your beautiful crystals without them breaking.
- If you want to grow an even bigger geode crystal, reheat the alum water and add another teaspoon of alum to the mix. Repeat the cooling process then place the geode back in the water. The water can also be reused for a new geode. The geode crystals will continue to grow the longer they sit in the water.
Can you make geodes with plastic eggs?
- Yes! We tried this method with three plastic eggs and while it did work, we found that the crystals grew significantly bigger on the real eggshells. I’m not quite sure why this is, but the plastic eggs make for a studier based and will probably last longer. Overall, the crystals were still beautiful despite the smaller size.
How do I grow big geodes?
- The more you reheat the alum powder water and place the geode back in, the bigger the crystals will grow. It’s important to let the geode dry and the water to cool completely before adding the geode back into the alum water. If you look at the photo of geodes you can see the blue geode is bigger than the rest. We grew this crystal for 3 days. The others were in the alum water for 12-24 hours.
How many geodes does this recipe make?
- The recipe will yield about 2-3 eggshell geodes, but you may be able to get a few more if the alum is reactivated. For some of the geodes, we split the alum water into two by placing 1 cup into small jars. The crystals didn’t grow as big, but this is a great way to make your alum powder go further and make a variety of colorful geode crystals. As long as your eggshell is fully immersed in the water, it will have the opportunity to grow crystals.
How to make eggshell geode crystals with alum
- 3/4 cup alum powder
- 2 cups water
- 1 drop gel food coloring
Prepare clean eggshell by applying a thin layer of glue to the inside. Sprinkle alum powder over the glue and set to dry.
Heat water in a small saucepan until boiling. Pour into a wide jar.
Stir in alum powder until it dissolves in the water. Add gel food coloring then let the water cool for 30 minutes.
Place the eggshell in the alum water for 12-24 hours. Then carefully remove with a spoon. Allow the geode to dry on a soft towel before handling.
This recipe calls for 3/4 cup of alum powder plus more for sprinkling on the eggshell. The alum water can be reactivated for a new geode by reheating and stirring in 1 tsp of alum powder.
If you decide to give this experiment a try, I’d love to hear how your eggshell geodes turn out!