Geode Cake Tutorial - Fool's Gold!

Geode Cake Tutorial – Fool’s Gold!

Sometimes you look at a cake tool and immediately an idea springs to mind. That very thing happened when I clapped eyes on the brand-new Diamonds in the Rough mold from Marvelous Molds. This crystal texture mold got the creative cogs turning and before I knew it, I was googling pictures of pyrite, better known as Fool’s Gold.

I’ve never made a geode cake before because honestly the idea of crushing up rock candy and painstakingly applying it to a cake never really appealed to me, and I like to try to bring something original to every design I create, which is why this mold really appealed to me,  there was just so much potential packed into this simple tool that I couldn’t help myself! I thought I would try to bring a unique take to the hot geode trend by showcasing “Fool’s Gold” rather than the traditional Amethyst that is so often seen, but if Amethyst is your thing, this mold would also work perfectly for re-creating that too.

I really wanted to make some pyrite rocks, studded with crystals, like the one shown here, I mean it’s gorgeous and it just screams, “make me into a cake!” The mold was absolutely perfect for the job, and I discovered the mold could be used in pieces just as effectively as in panels. I was able to build up the pyrite texture by tearing off sections of the mold and applying them to rocks and to the cake “geodes”.

During my experimentation, my four-year-old was assisting me, as he usually does, and he pressed the mold into a piece of rolled out fondant. I noticed immediately that the mold made a very convincing pebble/stone texture ..but more on that later!

As with all products by Marvelous Molds, I found the mold really easy to use, and versatile because texture molds are really some of the most versatile molds you can keep in your tool-kit arsenal, and playing around with them (outside of what they were immediately intended for) can yield some surprising results.

I hope you enjoy this easy Pyrite/Quartz Geode Cake tutorial, and if you’re not confident with using isomalt (as many of us aren’t) this mold would work perfectly as a stand-alone replacement for rock candy or isomalt pieces.


Fool’s Gold Geode Cake Tutorial

By Rebekah Wilbur



  • Diamonds in the Rough mold by Marvelous Molds
  • Confetti Already mold by Marvelous Molds
  • White Gum Paste
  • White Fondant
  • Black Fondant
  • Edible Glue
  • Small and Large Rolling Pin
  • Sharp Knife (X-Acto)
  • Modeling Tool
  • Selection of Brushes
  • Gold Luster Dust
  • Black Petal Dust
  • Clear Alcohol
  • Isomalt (Clear)
  • Measuring Jug
  • Crystal Mold

First, prepare the isomalt crystals. 

Melt isomalt according to the instructions and pour into prepared crystal mold. Allow crystals to set and turn out. To clear cloudiness, heat a steel knife and smooth over facets of the crystal. Set crystals aside. 

To make the pyrite (fools gold), condition some gum paste using a little vegetable shortening and roll into 3-inch rock shapes.  

Rebekah notes: the rougher and odd-shaped the better- as this adds to the realism. You will need to create 3 to 4 rocks for the design. 

Prepare the Simpress Confetti Already mold and Diamonds in the Rough mold by dusting with a little cornstarch, and shake away the excess.  

Condition some gum paste and roll to a thickness that is slightly thicker than the Simpress mold and slightly larger. Place the gum paste over the mold and press the paste into the mold, ensuring to really get the paste into the detail of the mold. 

Roll a small pin over the mold, the edges of the mold should trim away the excess paste. Turn out the mold. 

Apply some edible glue to the rock shapes you made earlier and drape the molded gumpaste over the rock. I started with the Confetti mold and then layered the Diamonds in the Rough mold over the top. Wrap the piece around the rock, taking care not to distort the pattern. Tear away any excess and gently blend the seams into the rock. 

Rebekah notes: You can repeat this step and tear the molded piece up into smaller jagged pieces and adhere in sections- this will build out the pyrite and give it a more realistic look. 

Using a sharp knife, add some texture to one side of the rock. You can also use the blunt side of the knife to press in-between the jewel facets, this will make the pattern stand out a little more and make the rock look more defined. 

While the rock is still malleable, add the isomalt crystals you made earlier; simply push the crystals into the paste until secure. It will start to look a bit like a porcupine.  

Once you are satisfied with the crystal formation, mix a little edible gold luster dust with some clear alcohol or lemon extract to make a paint and liberally paint the rock making sure to get in-between all the crystals. Allow the rock to dry. 

Once dry, using a clean brush, add a little shading by dusting some black edible petal dust in between the crystals. Leave the rocks to dry overnight. 

To prepare the cakes, fill and frost the tiers and set into the refrigerator to firm up. Use a sharp knife to carve out the geode areas. I carved out a section in the bottom and top tiers and a small 1-inch deep crevice around the middle tier. Once the cake is carved, cover with buttercream and place into the refrigerator to firm up.  To cover, marble some black and white fondant together, making sure not to over-mix (you should have clear bands of color) and cover the tiers.  

To Decorate the Tiers

Create several sections of the Diamonds mold using the same technique as earlier, and allow those pieces to set for several hours until firm, but not hard. This is an important step because using freshly molded gum paste to fill the geodes would become easily distorted and you would lose the definition of the mold. 

Tear the pieces into sections large enough to fill the cavities. Work in small sections if needed and use torn/uneven edges where the crystals meet the cake.  

Apply the sections to the cavities using some edible glue, and continue until the top and bottom geodes are filled and the carved sections around the center of the middle tier are covered.  

Paint the crystals with gold paint and allow to dry.  

Using a fine paintbrush and some gold paint, follow some of the veins in the marbled fondant to create some gold veining.

Add some shading to the geodes by applying some black petal dust. 

Rebekah notes: Make sure to shade both the crystals and the carved areas- this will make a big difference to the overall look of the cake. I also shaded at the base of each tier.

Secure the pyrite rocks you made earlier using a little edible glue. I added one to the top of the cake, a smaller rock to the top cavity, one to the bottom cavity, and one resting on the middle tier.  

Rebekah notes: Feel free to add more isomalt crystals and make sure to add a little shading to the base of the rocks to make them blend into the cake. 


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