This concretion is about 3cm. in diameter, it really looks like flint to me, I'm sure I could be wrong and I think I may be be seeing conchoidial fractures. The concretion formed around a nucleus that appears to have some structure, wether it's a fossil or not I don't know. I really expected the outside of concretion to sizzle, but it didn't, neither did the black flinty looking stuff nor did the nucleus. Any insight appreciated!