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
Comments
Nice specimen! If you just want it to be “polished” temporarily, for a photo, photograph it under water. Submerge it in a plate or bowl of water, then position your light so that there’s no reflection off the water surface.
Since it’s flat, you might try scanning it on a flatbed scanner. Maybe cover with dark cloth first.