The reason I am asking this question is because on one of my old pearl necklaces I have spotted when looking through a loop 2 of the pearls have tiny black things in them, near the outside of them. At first I thought it was dirt, but when I looked closer at them I realised that there was nacre covering it. The black things are near the drill hole and there is an indent in one of the pearls near the black thing. It is like the black thing was originally bigger and had been pulled away leaving an indent where it would have been with a little bit of the black thing still on the pearl where the nacre had covered it more thickly. I do not know if the necklace is natural or akoya, but I have not seen anything like this on any of my other different types of pearls that I have. the black thing is not deep in the pearls but more towards the outside of the pearls. a bit of the black thing is standing proud of the pearl and I was trying to get it off the pearl until I saw the nacre covering part of the black thing. I am puzzled by this. This is not something you can see with the eye but can be seen with a loop. I have a x20 loop so the black things is very small.