A few months ago I picked up these Clarus Glass Display Boxes from Crate and Barrel, and have since been filling them with some of my favorite rocks/crystal geodes. So far the collection is consisted of nine pieces and it's still growing. I am so glad that now I have a mini Smithsonian in our house, and it makes a great conversation piece.
(click on images to enlarge)
This is what they look like in the Clarus Display Boxes, still has room to grow.