Its a funny thing- you can actually buy powered eggshells in grugstore over here. Its supposed to be for people who has problems with bones and wants added calcium in their diet.
Guess, what, i decided to use eggshells in my smoothie. Ofcourse they have to be free range, or otherwise there will be lack of nutrients supposed to be there in the first place.
I guess i needed to mill the eggshells first, make powder and then use it in smoothie (now i can feel bits in it of eggshells), but i guess you do and you learn.
More info can be found here: http://nourishedmagazine.com.au/blog/articles/how-to-make-calcium-using-egg-shells