Hi; If you look at the first picture in the second set of pictures look at the top right corner of that picture. There is a small piece there that is darker in color than the rest of it. I see several spots like it. The thing about that dark spot is that it appears to have ridges. I think you ay actually have a true fossil there. I believe it may actually be a Dinosaur Egg.Don't do ANYTHING more to it. Bring it to your nearest University Archaeology Dept and ask them to positively ID it. Then come back and let us know what they said ok. It could be very valuable. Take care with it ok. I'm not kidding. I, we look forward to the results. PEACE:RONB
PS: I absolutely doubt it is just concretion ok. Good Luck. There are several other dark areas I see on it as well ok.