Are eggshells a traditional food?

[Nan'sMom]

I'm intrigued by the idea of adding eggshells to stock and/or to lemon juice for extra calcium (and I have added them to stock a few times) but I wonder:

(1) Does anyone know if they are a traditional food?

(2) Is the calcium from eggshells bioavailable?

Thanks for anything you have that sheds light on this!