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My 9 year old son found this and would love to know what it is. We were told its either a septarian nodule or fossilized bone.  The geologist was not familiar with Michigan and suggested I contact a geologist who was.

Any info would be great. Thanks

Hi Rachel,
Your specimen looks like a "septarian nodule".  

This type of rock is more properly called a "concretion" but the common name is septarian nodule.  It has to do with how it forms.  Both types of rock are sedimentary and are generated by heat, pressure, and mineralized water that infuse the layers of sediment after burial.  

A nodule is a replacement of minerals or a filling up of an existing void in the sediment.  It isn't really clear how it forms but generally, we believe that as the sediment turns into a rock, or after it has lithified, chemically rich fluids flow through it causing it to swell and create cracks both in the surface and the interior.  There is also a theory that the sediment in the interior shrinks causing the cracks.  

The name septarian comes from "septum" a Latin word for "partition".  The rock seems broken into pieces with all the pieces being glued together with another type of material.  

Here a couple of web sites that have images of septarian nodules.

Having said all that your specimen also looks like another type of sedimentary rock.  I have never heard it given a specific name but most geologist refer to it as ironstone or claystone nodules.  They are clay-rich sediment covered by iron rich clay.  The outside is hard while the inside is softer and powders well when scratched.  What seems to happen is that the sedimentary material lithifies but undergoes a process called diagenesis #chemical processes that continue after lithification#.  That causes the rock to swell and crack the surface.

This site shows both Ironstone nodules and septarians;

I think it is most probable you have a septarian.  The best way to find out is to cut it in half to see if it has the characteristic septa;

But I can certainly understand if you don't want to cut your rock in two.  Perhaps you have a rock club near you?  I have found that many local collectors have a very good knowledge of what can be collected in their area.  This site;

lists rock clubs in the US.  I don't know how current it is.

I hope this helps.


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C. Robert Reszka, Jr.


I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.


I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

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