Tiny white marks on battery

Of the iPhones that I've repaired (not just iPhone 6), I've noticed that a handful of them will have 1 or 2 very small white markings or dots on the battery itself. Looking closely they look like little "eruptions", around the size of the thread diameter of one of the screws. The edges around the white dot will be raised slightly, it kinda looks like a microscopic volcano or fissure. I seem to notice this more often when phones are particularly dirty. Are these tiny punctures caused by debris floating around in the phone, or are these marks something else entirely?