A very close friend of queen Giovanna's owned a turquoise and amethyst set made by Cartier in the 1950s. The set also had a brooch and a ring.
I saw part of this set on the daughter of queen Giovanna's friend about ten years ago. She told me that her mother had given the necklace to one of her daughter in law and that she, herself, had inherited one bracelet, the earrings, the ring and the brooch after her mother's death.
Sadly she sold these jewels a few years ago. The queen and that lady were very close during the war years and after, untill both their deaths. Actually, that lady was a very good friend of the whole bulgarian royal family.
And last detail, they (the friend lady and her daughter) were also invited to the Russian wedding in 1976.
I have no proof that those two sets are the same, but i think a loan, from that friend, is a very plausible theory.