Notice the scratches and the white dot. The scratches are from whatever it came in contact with. the white dots are from the crystals shifting or in some cases just the coating chipping off.
When I lived in Hawaii roaches were a big problem no matter how clean you were. I once found a packet of slides that they had eaten the color off in places. I imagine things like dust mites could also do that. They ate the glue off the back of postage stamps. They even ate holes through Old Tom's smokes.