Gray Stains on duck eggs?

[Basultooutdoors]

Today I went Into my duck run to find the usual egg. When I found the egg I examined it to see a large gray stain on one of the sides. I assume it is due to some calcium issue, however they are on laying feed.

I wondered if anyone as seen this before, and if it were safe to eat.

Thanks

 

[MerryMeridional]

Was it laying in something?

 

[Basultooutdoors]

It might have been in a little bit of water but am not sure

 

[Miss Lydia]

When you cracked it open did it look normal? those shell are made to be super protective of what's inside.