GeologyRocks

It looks man-made at first glance as it's very regular and symmetrical.  However, if you found it embedded in shale, it could be an ironstone or septarian nodule.  These are common in rocks such as coal measures and sediments of other ages and are concretions that form around a nucleus in marine conditions and become incorporated into the parent rock, sometimes recrystallising or being replaced by other minerals.

I would say a septarian nodule.

In all clays there are hard lumps, generally rounded in shape.  No one knows why (as far as I can acertain.).  Some, on diagenises, crack, allowing hypogenic fluids in.  Normally this is calcite in solution. The ones I sell are a pale brown matrix of clay and a lovely creamy yellow calcite, and where the two have met there is a rich dark brown band. I am not sure if this is a sort of baked margin or a chemical reaction between the calcite and the clay. 

John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant