Well, I was going to wait for a few more remarks but...
Cuttlegirl got it right (you'd make a good paleontologist)
Although it has the crystalline structure of a Belemnite, it is an internal mold of the siphuncle of an ammonite. It was hollow when buried and the crystals grew into the empty space, all the shell material, and whatever the connecting ring of the siphuncle was made of has been desolved, leaving only the crystallized internal mold. Below are 3 pics of the last three chambers of a large Prionocyclus macombi. You can see the same crystalline structure growing toward the center of each chamber (best seen in the middle pic). In the first and last pics you can see the siphuncle, an end view in the first pic and 2 segments and the mold of a third in the last pic (thing 1 is on the left).
Sorry for the delay Kevin. Been off for a few days, at a conference on the British Bronze Age(!) and helping out excavating at a local mill where a mysterious floor has been discovered underneath the existing 19th century mill.
Anyway, that's pretty interesting. I would certainly not have guessed that was a crystalline cast of siphuncle from an ammonite. I knew it wasn't a belemnite, but beyond that I would have been stumped. How unusual!