This is just plain weird: retrieved an egg yesterday that has a patch on its shell made of hundreds of small spheres. They can be scraped of with some effort but are pretty tight to the shell. Anyone have experience with this?
Calcium deposits. Many of those weird eggshell variations are caused by genetics. Sometimes there are underlying issues, but what you describe I wouldn't worry about. (Thin and rough-shelled eggs are more cause for concern.) For more about egg laying, you might want to read my blogpost. http://www.hencam.com/henblog/2012/02/brown-green-blue-white-chicken-egg-color-the-real-story/