Fossil Lobster Burrow with Septarian-Like Cracks

This is not a septarian as such, it's a fossil lobster burrow, fragments of which are quite common here. I cut it in a cross-section and polished it with the help of some Paraloid B-72 as it was very soft. There are threads and patches of black pyrite running through it, as well as cracks that seem fairly similar to a septarian nodule.

5.5cm