Time for a pretty picture, don't you think? This is a cut slice of a septarium, also known as a septarian nodule. It's a kind of geode with a network of cracks rather than a blobby hollow inside. Often, like this one, they're completely filled with mineral crystals. See it close up in the Geode Gallery.
I should probably polish this for a better appearance, but I'm lazy and don't have the equipment anyway. Can I just varnish it or something?
Septarium Geology Guide photo