Make Your Own Geode Cake

Earlier this summer, I received a request to make a geode cake. The request came through my online order form and the customer attached a link to a video. In the form, she said she could make her own rock candy for the cake. I thought the link was for a “how to make rock candy” video so I didn’t watch it, thinking she would be the one making the rock candy. Instead, I planned a geode cake image in in my head and called her back.

Did you watch the video?

She asked me as soon as we got on the phone. We started talking and it was immediately clear that my image and her video were worlds apart! It was time to fess up that no, I had not in fact watched the video. My vision was of a circular (think half a sphere) cake while she was describing a tiered cake with a portion cut out.

Calling all video learners

If you are interested in watching the step-by-step how-to video, please check out the video that inspired this cake. It takes you through every bit of what you need to do for the cake. And then if you want to read on for our pictures you can.

For those who prefer text and pictures

I don’t exactly think I’m impatient, but I’m creative: I struggle to follow other people’s step-by-step instructions, no matter what genre they choose for delivery. Even so, my customer wanted to work with me to make the cake so we did watch the video. Once more, my son has just started as my new social media intern. He was VERY excited to do a live stream of our session. So, during our time making the cake, Xander had the camera catching our every move! After the fact he enthusiastically told me that we had 3 viewers. My daughter quickly piped up that one of them was her! So, needless to say, our video was not very popular.

My step-by-step is a summary of what we did and it includes a few pictures. For a full how to experience, definitely check out the video. But, if you are keen to take the directions and just run with it, this list will help you hit the ground running!

How to make a geode cake

First select your cake flavor(s) and sizes. I used a 6″ round for the top cake and an 8″ round for the bottom cake. Heidi’s husband (the recipient of this surprise geode cake) loves chocolate and raspberry, so she chose a chocolate cake with raspberry filling. The raspberry filling adds another layer of challenge to this cake that was pretty fun!

Bake, fill, frost and chill your cakes.

Select your fondant colors. The exterior of this cake is covered in a marbleized fondant. We chose to use blue and white, the colors from the video.

To marbleize the fondant, mix a generous amount of white fondant with blue fondant. Massage the fondant like you would play with play-doh as though you were trying not to mix the colors too much. Stop when you reach a color combination that you enjoy.

marbleize your fondant

Roll out your fondant to cover your bottom cake. You will want your fondant to be large enough to cover the bottom tier completely. Our 8″ round cake was about 4 inches tall, so we made sure to roll out a disc of fondant about 16 inches in diameter.

Roll out fondant to the marbleized consistency you like

Cover your cake with fondant. Gently lay the fondant on the top of the cake and smooth the edges using a fondant smoother or your hands. Trim the excess fondant and add it to the other fondant you will use to roll out to cover the top cake.

Heidi rolling out the fondant

Use dowels or straws to provide support in your bottom cake. To make sure you place the top cake where you would like it, lay a 6″ parchment paper down on the prepared 8″ cake. Mark the exterior edges of the parchment. Then, place 4 dowels or straws in the middle of the bottom of the 8″ cake. I prefer to use straws because they are easy to cut, but either straws or dowels work very well. Because you need to cut away a portion of the 6″ cake base, do not work with plastic plates or the SPS system for this cake.

The dots are the marks of the exterior edge of the parchment paper. This tells us exactly where to place our top cake.

Repeat the entire process with the fondant, this time rolling your fondant in a 14 inch circle to cover your 6″ cake. Place the top cake on the bottom cake, affixing the two tiers together with a little bit of buttercream frosting.

Now it is time to cut away your cake for the geode. The added bonus to this is that you have a built in snack! Carefully trim out a pretty big portion of cake from the top and bottom cake. You will have to cut through the cake board for this and remove it. I used a medium sized knife for the cake cutting and switched to strong kitchen scissors to cut away the cake base.

Cut out a portion of the cake for the geode.

Add your rock candy

Heidi said she thought she could make the rock candy. However, it was the end of July in Central New York during a very rainy summer – think VERY HIGH humidity – so not great for crystal formation! She was heading out to California for a week, but thought she could start them and have her daughter check in on them. I told her to report back in because I could buy rock candy if we needed to. I was in the mountains in Oregon for a running camp that my sister hosts for the weekend, but I could find a candy store online and place an order.

Heidi reached out that the candy wasn’t crystallizing. So, from my mountain lodge one morning I searched for rock candy online. I came up with several options. After a few tries, we ultimately agreed on blue and white rock candy gems and a bag of assorted multi colored crystals. That would give us size variations to work with. I bought the candy on Candy Nation, a bulk candy site. I was incredibly pleased with the ease of purchase and fast and affordable (not free) shipping. With a pound of each color, we ended up purchasing $26 of rock candy for this cake. Luckily there was a fair amount of it left that Heidi could take home to her family!

Before you begin to place the rock candy, frost the area that you cut away. Then, use tweezers and your fingers to gently place the colors, working from the inside out. Heidi and I were a great team! She sorted colors and I placed them on the cake for the most part. When all the colors were sorted we talked about the bottom of each cake. We agreed to put gems along the side of the bottom tier. The inspiration cake had rock candy along the bottom of both tiers, but I thought it would be nice to have a fondant bead border instead of so many crystals. Heidi agreed to the plan.

After laying a thin coat of frosting we each took half of the bottom tier to and placed the rocks. I added the bead border while she filled in gaps on the top tier. To make the bead border, simply make little balls of fondant and affix them to the top tier with either piping gel or frosting. I used the homemade piping gel that I made for the cake.


The inspiration cake called for edible gold leaf to use as the border for the edge of the geode. I felt like while this was super cool, it was another expensive thing to buy for the cake. I had gold luster dust. We mixed that with lemon extract and painted it on as a border. It worked to create a nice edge at a fraction of the cost of the edible gold leaf.

Decorating the top

The party was a surprise 60th birthday for Heidi’s husband. We discussed ideas for the top. Should we do a big pile of rocks like the model cake? Should we include the number 60 somehow? His name? We decided to highlight the number 60! I used large number cutters to cut the numbers out of fondant. Then, I brushed a thin layer of piping gel on the numbers. I laid the numbers on the cake and Heidi placed the candy rocks on top.

…and that raspberry filling

At the start of this post, I mentioned that you need to pick your cake flavor, right? Heidi went with chocolate and raspberry. This is such a delicious combination, but it did present an extra challenge for this style of cake. When I prepared the cakes, I had to designate a portion of each cake to have vanilla buttercream filling, so that when we cut away the cake it didn’t fall in on itself causing raspberry filling to ooze out. This was easy enough; simply select a little area to fill with buttercream instead of raspberry and mark it.

I was pretty pleased with myself as I went to cover the bottom tier with fondant. And then, I spun the cake around when it was covered only to realize that I no longer knew what part was filled with vanilla buttercream. Yikes!! I had a guess though, so I marked it and decided we’d hope for the best.

Heidi had a suggestion for the top tier that worked GREAT! What if we placed little toothpicks at the bottom of the tier so that when we covered it with fondant they’d peek out and we’d see which portion was filled with buttercream? Yes that would be fantastic. WE covered the top with the fondant and knew exactly what part was safe to cut away. As long as I had guessed correctly for the bottom tier we’d be in great shape. And as luck would have it, I had!!

The lesson here – use the toothpick method. Just make sure to mark where you want to cut away and line them up properly and you can use any frosting/filling combo that you want for this cake.

Notice the toothpicks here to mark the edges of where we can cut out.

The finished cake

In about 4 hours you have the perfect surprise geode birthday cake!!

60th Birthday Geode Cake


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