I have never encountered a shell as such. It may be a slight abnormality in the shell production,,, when the ink is applied.
If you crack the egg, and it looks fine,,,,, then just eat as the rest. I would.
If you are skeptical about eating it,,,,,, then cook it up, and feed back to your flock.
I eat my chickens' eggs. Occasionally I do run across a hidden nest with eggs in it. They are not fertile, since I keep hens only.. A chicken went broody and trying to raise her own family.

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I take all of those eggs, and boil/cook them. I then feed the eggs back to the flock. Shell and egg contents just smashed when I step on them. It is like a recycling program. I don't want to eat questionable eggs that I have no idea how long they have been hiding there.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
