This is a crystal of the metal bismuth. The rainbow colors result from a very thin layer of oxidation on the surface. (Alchemist-hp, Creative Commons License).
Bismuth Geode/ bismuth that has been grown and cooled in an eggshell! Apparently bismuth is one of the easiest crystals to grow yourself. I found instuctrions at About.com. Bismuth does not naturally occur in geode form. What fun to create your own dragon's egg!