Hm... that, only the pearl knows by now, I would think.
Between the long 'cucumber' shape of the pearl in the painting and the fat pear of the Liz necklace, it must have been chopped off rather then just peeled! At least to me, the difference of shape seems very large.
Now, obviously the painter may have taken some licence, or it may be that the pearl was simply caught from a different angle and the difference between the two images is not material... Just writing what I see
However, Antonio Morro owed his numerous international royal commissions particularly to the accuracy of his portraits - that's pretty much all he did for a career, and was knighted for achieving particularly that one (part of a series after the same sitting). Basically, I'd bet he took as much care of the queen's outstanding jewel as of her face.
That's pretty much what I had in mind.
I sort of remember that there were doubts about the attribution of the Liz pearl before. Do you remember who-what-when by any chance ?