Geodes develop inside bubbles in molten rock. As the rock cools, the bubbles harden into hollow spheres with minerals inside them. Moisture inside the bubbles begins to dry out, leaving these minerals behind in the form of very hard, colorful crystals. Most amethysts form this way, as do some types of quartz. Since geodes take decades to grow and harden, it's impossible to watch them form in nature. Instead, grow geodes at home with just a few simple craft items.
Add two-thirds cup each of plaster of Paris powder and warm water to large bowl. Add one-fourth cup of alum to the mix and stir until it becomes the consistency of dense pancake batter. The alum in the plaster will help the geode crystals grow.
Spoon one-half cup of plaster into each of four bowls. Gently and quickly smear the plaster up the sides of the bowls, moving down the line of bowls as you work them. If you work just one at a time, the last bowl will harden before you can shape it.
Continue smearing the plaster up the sides of the bowls until it begins to harden. When the plaster remains in place, let it cure for 24 hours.
Mix 2 cups of alum powder with 3 cups of very hot water. Stir the mixture until the alum dissolves completely. Add more alum, one tablespoon at a time, until the water stops dissolving the powder.
Pour 1-1/4 cups of the alum solution into each of your plaster shells. Add a few drops of food coloring to each shell so you get colored crystals. This also makes the crystals easier to see as they form.
Gently place the bowls in a dark, warm, dry place. After a week, you should see seed, or very tiny, crystals begin to form. More will grow and form until the water evaporates. This may take up to a month.