Septaria

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Septarian concretions are often regarded as geological curiosities. Imagine a lake bed that dries up and cracks. Then is covered up and eventually fossilizes. And the old cracks eventually fill up with other minerals.

This is not really an accurate description of how septaria form, but it is a pretty good analogy.
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This link for wikipedia does a far better job of explaining what they are and how they form: http://en.wikipedia.org/wiki/Concretion

Sometimes they have calcite in the cracks, and sometimes this calcite contains trace elements that makes it fluorescent. I have several septaria that are fluorescent, including a large one from the South of France.
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This one on the right is from the far south of Utah, near to Ordervillle. Clik on the picture and it will enlarge. Orderville is east of the Zion National Park, down near to the Utah-Arizona border. This septarian concretion is about 6 inches in diameter. I bought it last weekend at the mineral show in Brussels, Belgium for EUR 12- which is about $15-.
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