Egg with grainy surface, what causes it?

[50-45-1]

Hello everone,
Yesterday i collected this egg with a grainy surface. Does anyone have an idea what causes this. The other 2 eggs are my normal eggs i gather every day.
I dont know who laid this egg but i do have brought home some new hens this spring. All my birds are over 3 years old.
The new to me hens are: 1 hen who was wandering the streets of my town this spring for a month. 4 more girl off craigs list who the owners were tired of having chickens.
I free range during the day, feed well rounded chicken feed, have a container if oyster shell and a container of grit avaliable at all times.
Any ideas?

 

[crazy4ChickensNducks]

It’s just calcium deposits

 

[sumi]

Calcium deposits, yes. There is a list of possible causes of that and other egg problems here: Common egg quality problems

 

[aart]

Probably just a one off, shelling gland glitch...
...I'd wait to see it if continues before becoming concerned.