What you see: Translucent, watercolor-like smudges on your eggshell
What it is: Mottling, or a place where moisture is trapped in the shell
Eat or toss: Eat! This is a cosmetic issue (if it's a spiderweb pattern, however, you may be looking at a crack so check out this post)
Calcium carbonate crystals make up the bulk of an eggshell, which, in a standard store-bought chicken egg, is a solid white color. But what’s up with the gray, watercolor-like pattern on some eggshells?
“They are areas of the shell that are slightly thinner or have a slightly different calcium carbonate crystal structure, which allows moisture to get trapped” said Ken Anderson, a poultry specialist at North Carolina State University.
So, that gray, watercolor look? It’s actually a tiny bit of water! But the egg is still intact so it’s OK to eat.
Stressed or older hens are more likely to produce eggs with a mottled, “windowed” or spotted pattern, according to a piece co-authored by a University of Florida poultry veterinarian whose name really is Gary Butcher.
There’s nothing immediately unsafe about this egg, but watch out for a similar grayish pattern in a spiderweb, starburst or shatter-style pattern. That’s an egg with very fine cracks that have allowed in moisture. Such an egg isn’t necessarily unsafe (the shell, but not the inner membrane has broken), but the odds of something problematic getting in have inched up a bit, so use it before the others in the carton and make sure you cook it to 165 degrees Fahrenheit, or are positive that the entire egg has been held at 145 degrees for several minutes—that’s how you kill salmonella.
Kenneth E. Anderson, Ph.D. Professor, Poultry Extension Specialist. Director, North Carolina Layer Performance and Management Program. Prestage Dept. of Poultry Science. North Carolina State University
Web exclusive: Eggshell spotting/mottling: What is it and why does it occur? Gary D. Butcher and Richard D. Miles. WattAgNet.com. July 6, 2007.Egg Shell Mottling and Hatchability. R. Keith Bramwell, Extension Reproductive Specialist, Center of Excellence for Poultry Science - AVIAN Advice newsletter - University of Arkansas. Published under Poultry Industry, by Engormix. June 27, 2008.
Determination of the quality of stripe-marked and cracked eggs during storage. Yu Chi Liu, Ter Hsin Chen, Ying Chen Wu, and Fa Jui Tan. Asian-Australasian Journal of Animal Sciences. July 2017.
“Maintaining Eggshell Quality.” Lokesh Gupta, Regional Technical Manager, Avitech. March 14, 2008. The Poultry Site.
Prairie Fare: Are Runny Eggs Safe to Eat? Julie Garden-Robinson, North Dakota State University Extension Service food and nutrition specialist. April 7, 2011
Eggshell mottling, does it affect incubation results? Lotte Hebbink. November 2018. Pas Reform.