This image of an amazing Amethyst geode is from a Squidoo article called "Grow Your Own Crystal Geodes" by Dr. Anne Helmenstine. Although this geode is natural rather than homegrown, I still think it sounds really fun to grow your own geodes!
Bismuth Geode/ bismuth that has been grown and cooled in an eggshell! Apparently bismuth is one of the easiest crystals to grow yourself. Bismuth does not naturally occur in geode form. What fun to create your own dragon's egg!