Churt Nodule or Possible Bone Fossil
Unknown excavation source
Approx. measurements: 4" x 4" x 4", weight not given

A chert nodule in sedimentary rock is a distinct, often rounded or irregularly shaped mass of a hard, fine-grained, siliceous sedimentary rock. Found embedded within sedimentary rocks, most commonly limestone or chalk, but also in shale or dolomite. They often stand out due to their hardness and resistance to weathering compared to the host rock. The examples are gray, white, black, brown, or red, and depends on impurities or the environment. For example, iron oxides can impart red or brown hues, while organic material may cause darker shades. Some chert nodules do preserve fossils of small marine life (sponges, echinoids) with exceptional detail due to the fine-grained silica replacing the original material.

This example does have a distinct area that does appear to be a different make-up from the surrounding limestone. Yet, from images alone- I do believe it to be geologic and not biologic in formation. I am one opinion, if you would like to research your example further; I do suggest an in-person assessment. You can schedule an assessment with a regional University or Field Museum Geology Department. You could also contact your local library to discover more resources for research on churt nodule formations or when geology and rock and mineral shows are in your area. There are also paid services with regional auction houses and appraisers for a more thorough, in-person, assessment.