It tells you which side is "up". If the egg is turned over by accident the baby gecko will drown in the egg. It's just done with a sharpie, which is non-toxic. You mark them when you find them in the lay box.
Yup, or I use it for marking eggs within the SIM. Since I will have clutches from different females in the SIM I like to identify the female and the number egg. You can see examples under my eggs incubating ( <--- under my personal info)
So marking with a typical black sharpie is just fine and won't harm/bother the hatchling huh? Have you / has anyone used a pencil or other implement to mark eggs to know which ones came from who?