A Septarian Nodule starts as a mud ball at the bottom of an ocean. Often forming around a fossil or organic debris.  As the mud hardens (a chemical process rather than by evaporation) cracks form and are filled by other materials, often calcite, which gives an interesting contrast between the dark mud stone and the lighter colored calcite.

There are Septarians found around the world and several locations in the United States.

Michigan - http://www.mcrocks.com/ftr08/HarringtonApril2008.html

Ohio - http://www.vasichkominerals.com/Lamereaux1.html

Kansas -http://www.kgs.ku.edu/Extension/KGSrocks/concretion_def.html

Utah - http://www.berea.edu/geologymuseum/galleries/tour/12-septariannodules.asp

 

 

For more information try these links:

http://stoneplus.cst.cmich.edu/septarium.htm

http://www.rocksforkids.com/R&M/concretions.html#Septarian_Nodules