Interesting. The clasp does look like it could be a sapphire in the close up.
I'd buy it out of curiosity, price depending.
ETA: I have a hoard of Mississippi River Pearls and while it's difficult to tell from the photo, they certainly could be the real deal. Sellers photos leave a lot to be desired. Also the stone appears more like a blue spinel - I have a MRP brooch that has a very similar looking stone and it's a spinel.
It ships Tuesday due to seller’s travel.
I'm excited for you also, Aerinha! A very interesting, vintage strand with lovely clasp! About 10 years ago I purchased a double strand of Mississippi River pearls on ebay, as posted here on the forum. My pearls are still awaiting inspiration and a special clasp, as the original clasp had a glass stone that was broken, so not reusable.
Good question, my pearls definitely look best with cream color thread, probably yours will also. The thinner thread, size #40 will be the best if your strand has small holes. My end pearls are so small I won't be able to double back, so probably will end with some gold beads. Like you, I prefer knotting between each pearl! Excited for you to receive your necklace!