MOP is heavier like ctg4570 says and when you handle the real pearl as you casually shuffle the two together they sound like heavy glass clicking together. It's a very dense material. Plastic sounds dull.
Mother of Pearl,,,,or Mother of Plastic,,,,,,
Just looking at a picture is very tough to tell as some of the synthetic material is very good.
Are those for a Spanish Star/llama or are they Colt??