Real pearl necklaces or off to the yard sale bin?

I just got these three necklaces at auction. Full disclosure, I know nothing about how to accurately tell if these are real or costume. They sink in water, they are not knotted between the beads, the clasps are not precious metal. The third strand (the one with the tiny pearl on the clasp) has a slightly more yellowish cast than the other two. When I look at that one under a loupe it seems ever so slightly uneven with tiny tiny pits.

Your thoughts?

Lynnie

 

did you scratch them with your front teeth?

I'm thinking..... paste or some such thing...

 

I did do the scratch test, and POSSILBLY the third on passed.