Fondant outer rock,color like marble or rock, then shape into hollow geode. For the crystalized part, you just make a sugar water solution. We used 1 c. of water and 2 c. white sugar. Cook it in a small pot until the sugar is completely dissolved. Add in any coloring you would like to use. If you are doing different colors, divide them out first. Pour the sugar water into the hollow of the rock part. Wrap with foil, let sit about 3 days, carefully unwrap and pour out excess sugar water.
Sugar Crystal Sticks. A fun way to demonstrate crystal formation to students (GEOLOGY). Turn it into a competition by giving the instructions/link to the class so they can make the sugar crystal sticks at home. After one or two weeks get the students to bring in their crystals. Give prizes for different characteristics e.g. longest stick, largest crystals, most creative shape, most interesting colour, etc.