Here's a video that shows you how to make the cake:
White cake (or cake of your choice)
White rum or vodka
Black food coloring
Rock candy (ranging from white to dark teal/blue, or colors of your preference)
1. Cover cake in smooth layers of fondant.
2. Cut the geode shape out of the side of the cake.
3. Spread buttercream frosting all along the inside of the cut
4. Mix alcohol with food coloring until it reaches the desired shade of gray. Paint a thin dark line around the edge of the cut.
5. Apply a row of piping gel in the center of the cut.
6. Press rock candy into the piping gel, starting with the darkest shade.
7. Continue laying out rows of piping gel and pressing in the rock candy, applying lighter colors and smaller pieces as you move farther out toward the edge of the cut.