Create the Geode Cupcake by marbling white and black fondant, rolling it out, and cutting 3” circles. Attach to a buttercream-iced cupcake.
Color white rock sugar with turquoise shades using food color gel and grain alcohol. Note, the grain alcohol will evaporate and leave behind the vivid colors.
Brush piping gel onto the marbled fondant topped cupcake and place colored rock sugar on it, focusing darker rocks in the center and lighter ones around. Ensure no gaps for a seamless geode effect.
Outline the geode with more piping gel, sprinkle it with white sugar, and then use a mixture of gold luster dust and grain alcohol to paint a golden border around the sugar outline.