To me the gems look too "flat" to be pearls. She could have set the tiara with turquoises because that was what she had then to use and as such she thought it was a good idea to borrow the necklace from her aunt as it would make a good match for that event. Because I do think that it is the same necklace worn by the Duchesses of Gloucester. It would be very strange to have two exactly identical necklaces within the family.