Hullo everyone. This was picked up on a beach in East Ruston, Norfolk, UK. The parent rock is flint, I think, as is most of the rock on that beach.
The rock carries a textured feature in a hollow. I didn't have anything other than that 18mm (~3/4 inch) 5p coin for scale.
The feature appears to be mineral. It can be chipped out with a blade and the fragments are gritty.
It's probably humdrum but I'd like to understand what I've found so thanks to everyone for looking.