A tool to break open geodes, a geode "guiilotine" - assembled from 4mm laser cut steel. The blade inserts are roughly sharpened with an angle grinder - doesn't want to be knife sharp, since geodes will just blunt them., and that's wasted blade...

Put the geode in the "jaws", place a piece of wood on top of the guillotine - and smack it with a hammer.

Pop, goes your geode.

Here's a video of it in action : https://www.youtube.com/watch?v=OsRIusou4Zo