Brief aside: despite having a Ph.D. in geophysics and a Bachelor's in Earth Science, I have never actually taken a class in geology. Most everything I know about actual rocks (as opposed to theoretical rocks) I picked up by osmosis because my mother ran the Geology Museum at the Colorado School of Mines for two decades. I would hang out there after school and dig through the specimen drawers. My knowledge of mineralogy is limited to rocks that are (A) pretty (because those are the ones that are best to look at), or (B) poisonous, (because those are the ones that your mother tells you "wash your hands when you're done with that" about).
But anyway, Google said to get a chisel and just score the geode around its circumference until you get a crack, so we did exactly that, and after a goodly chunk of forearm-strengthening exercise, we got a crack. A couple whacks more and it split right in half.
The crystals inside were a sort of bluish-gray, very small and delicate, almost furry. I feel like I ought to say something insightful and/or metaphorical about it, but mostly, there was a rock, and we split it open, and it was pretty inside. Now it's sitting on the Nevilles' mantel in the mancave.
Also, six-year-olds are highly distractable creatures.