Calcium deposits - look like bug eggs - on duck egg shell?

One of my Ancona duck hens has laid an egg the last couple days that appears to have either funny calcium deposits or some sort of bug eggs on the egg shell. I'm guessing/hoping it is just calcium deposits, but they are tiny little circular bumps in a single layer in small clumps scattered over the shell that if they appeared anywhere else I would assume they were bug eggs. They are easy to scrape off with my finger nail. I think they are hard - I should have tried to crush them and see if they powdered up. I'm sorry I don't have a picture. If it happens again tomorrow I can take a photo, but I already cleaned the two from the last two days.

Has anyone else ever seen calcium deposits like that before? I want to make sure they are safe to eat (that they are not in fact bug eggs). Thanks!