reltucdog on ebay does a beautiful job with sepatarian nodules, including the huge pictured above. You should check out their stuff on ebay. This is their description of Septarian Nodules:
Septarians Geodes were formed approximately 100 million years ago when the Gulf of Mexico reached what is now called Mount. Carmel in Southern Utah. Decomposing sea shell life,(which are similiar to Ammonites) were covered up by ash from a volcanic eruption. The fossils had a chemical or molecular attraction for the sediment around them, and as the tides and waves kept rolling them around they formed into mud balls As the ocean receded the mud balls were left to dry and crack. Because of their bentonite content they also shrank at the same time trapping the cracks inside. They were then covered by sediment. The ocean returned depositing more shell life above them. As this decomposed calcite from the shells was carried down into the cracks in the mud balls, calcite crystals formed. A thin wall of (shale) exteriors from the calcite centers. Because of this dividing wall (septum in Latin) the geodes are called Septarians.
