The necklace graduates, which you would not tend to see with imitation pearls. The overtones of some of the pearls are a bit greener than others, and I see what look like blemishes on the 5th pearls from the clasp in the first photo of post #2, unless those are reflections. I'm leaning toward high quality, well-matched Tahitian pearls, but by all means do the "tooth test" Mtummybear!
Edited to add: I have stopped recommending rubbing pearls against teeth (“Tooth test”) Teeth are harder than pearls and can scratch them.
Instead, rub 2 of the pearls gently together, after wiping them clean with a soft damp cloth to remove any grime.