We were at Lagoon this weekend and in Pioneer Village, they sell Geodes for 4 and 6 bucks each. My son and I decided to give them a try.
I grabbed my angle grinder with a diamond blade and it cut them open fairly easily.
I could tell the big one was hollow because we could hear some pieces rattle around inside.
Warning on the volume, the grinder is loud.
https://www.youtube.com/watch?v=eORI-FjDSDo