MOP stands for "mother of pearl" and jewelry made with this material seems to be getting more popular!
Mother of pearl is a smooth, iridescent substance that forms the inner layer of the shell of some mollusks, especially oysters and abalones. So..."pearls" have formed inside a shell in their "pearl" shape. Mother of pearl, on the other hand, is made of the inner layer of the shell itself.
Freshwater pearls are so pretty but I also see the allure of mother of pearl, especially in that they can be quite large and unusually shaped.