It kind of looks like a monochromatic map of the world.
However . . . I looked in my "Chicken Health for Dummies" book, and under "Eggs that have a sandpaper texture, lumps, or chalky coating on the shell" Possible causes: Stress or Young hen or Hen delayed in laying egg. Solutions: Stop stressing your hen out! -- oh, I think they worded it, "Eliminate Stress." Also provide more nest boxes. (Mine insist on using the same box!)
I also ran across something for everyone with layers to keep in mind, especially those who've had hens lay eggs with a soft shells : A laying hen needs more calcium in warm weather (they eat less and therefore get less calcium in their diet). They also need more calcium as they age. A calcium source (like oyster shell) should not be mixed with other food to force or encourage hens to eat it. Calcium should be offered separately, free choice, so hens can take as much or as little (or none) as they want/need. A vitamin AD&E powder can be added to drinking water 3x/week. (From "The Chicken Encyclopedia")