DIY Geode Cake Tutorial: School Holiday Project

Here is a perfect indoor, multi-step activity to occupy the little ones, while slipping some hidden science into the mix.

  • Difficulty: Medium (Handling Hot Liquid and Stove Use Required)
  • Time Required: 2-3 weeks (for crystal), 2 hours (for decorating)

Geode cake is popping up all over the Internet, but it is imitating a natural geology phenomenon in the world. Taking the same principle, you can replicate this ancient science in an edible masterpiece.

This cakes take a bit of planning (best started 2-3 weeks ahead) but can be done with most common pantry items and tools. The result is worth the wait.

Growing the Crystals

*****This must be done by an adult. Sugar syrup is extremely hot (hotter than boiling water)! Even adult needs to be careful!*****

We will use the process of “crystallisation” to make our edible crystals.

You need…

  • 2kg of sugar
  • 500g of water
  • Skewers
  • Food dye (optional)

To Make the Crystal…

Put the water and 3/4 of the sugar in a pot. Heat the mixture until the sugar have melted.

Add half a cup of sugar at a time until you can see sugar crystals not melting at the bottom even when you stir.

Add food dye if using and stir

* Be Careful: Very Hot! * Pour sugar syrup into heat resistant containers e.g. glass. Let cool.

Wet the skewers with a little water and shake off the excess. Sprinkle sugar onto damp skewer.

When syrup is cool, put sugared skewer into the sugar syrup without touching any sides or bottom of the jar. If needed, clip some clothes peg on top to help the skewer stay straight.

Cover with paper towel. Wait for crystals to grow….for 2-3 weeks.

Remove skewer with grown crystal from the syrup onto sieves to drain and dry. Bend the skewer to get the crystals off .

If you are not doing the cake immediately, store the dry crystal on some baking paper in an airtight container.

Assembling the Cake

You need….

  • Dense Round Cake of Your Choice (e.g. mud or butter cake)
  • Enough Buttercream to Cover Cake
  • Gold and Black Food Gel Colouring
  • Sugar Crystals You Have Grown
  • Black Powder Food Colouring (optional)

Mix in gold food colouring to your buttercream. Put enough just so its a creamy colour.

Add black food colouring to your buttercream. Mix at low speed for a few seconds – just until its marbled. We are looking for streaks of cream and grey.

Trim the cake top edge so that its round up the top. Don’t worry if its a bit rough, no stones are perfectly round!

Have a look at how many sugar pieces you got. Cut a wedge out of the top of the cake that can be covered with what you have in one layer. (Hint: Start small – you can always carve away more!)

Cover the entire cake with buttercream. Dab the butttercream cream with the tip of spatula to create rock like texture.

Start from the middle of the wedge, push a piece of sugar crystal into the buttercream. Work your way out in concentric circles until you meet the edge of the wedge.

To finish, sparingly sprinkle black food powder colouring on top.

Voila, enjoy and wow people with your edible crystal geode!

Level Up…

If you want to make the geode super realistic, you can grow 2 different colour crystals. Just pour off half of the sugar syrup into one container when boiled. Add more colouring to the remaining syrup in the pot to make it darker. Pour the darker syrup into a separate container with its own sugared skewer. Leave for 2-3 weeks. When placing the crystal onto the cake, start with darker shade in the middle and continue with increasingly lighter crystals on the outside.

How are Geodes Formed?

Geodes starts off as air pockets when rocks are formed. For example, lava running off a volcano may run and cool in such a way that gaps are trapped inside. (Like gaps that gets trapped when making a trifle). Water with lots of mineral gets into those air pockets. When the water contains too much mineral, crystals starts to grow in that air gap. Just like when we put so much sugar in the syrup, the sugar wants to escape onto our skewers. So a few hundred/thousand/million years later when a miner comes along and breaks open that rock, it contains a beautiful crystal inside.