We have this neat little shop in our town called "Cool Stuff" and he loves rocks and minerals. He has a number of different minerals to sell, and one of them is Geodes. He said the trick to find one that would be hollow was finding one that was light compared to the others for it's size. Some Geodes will be solid inside, but I think the prettiest ones are hollow with neat crystals growing inside. So this is the rock we picked. We found what looked to be a natural line around the rock, and took turns pounding on it. I think we did a pretty good job. :) We picked good, our rock was absolutely beautiful. :) Now we want to try growing our own in empty egg shells. We hear you can mix water and epson salt and it grows in maybe 30min. to an hr. :) Add a couple drops of food coloring and you'll have colored crystals.