This is a great project to use up all those casting soap scraps you've been accumulating. Or you can, of course, make soap especially for this project. To form the geodes, you will chop casting soap into two sizes. You'll tint the little chunks and form them into spheres of two layers. Then you'll dip-coat them with darkly colored soap. This is a very messy project, so you may want to wear latex gloves.