Where was the Necklace Tiara borrowed from?
I really don't know
This is how the necklace/tiara looks like in whole:
https://i.pinimg.com/originals/a7/02/75/a702759f74103b7b822228ea35ab7d67.jpg
It must have sold at some point and this picture could from an auction. At least it looks like that. If we could find the auction, we could have had some more information about the provenance.
Btw, before the marriage Princess Aspasia and Alexandra were financially supported by Sir James Horlick, 4th Baronet, with whose help they bought Garden of Eden Villa in Venice, so it could be Horlick baronets piece.
The pearl necklace and earrings in the previous photo were 18th birthday gift from her mother and I think that today Crown Princess Catherine wears them.
But, on the other hand, she was already married at the time she wore this tiara, so it could be borrowed by any of their wealthy friends of more probably from some jeweler.
She didn't like to wear any jewelry that had been previously worn by any lady who was in their circle of friends. For example, when she decided to give back the Russian Emerald parure to King Peter II to sell it (which was her property at that time), as he needed money, she stipulated that it could not be sold to any person, but just to the jewelers as she didn't want to get into any awkward situation and wanted to prevent that any lady might wear her jewels at an event that they might attend together in the future.
The only thing she borrowed from a person was her wedding dress, as they married during the war, in a hurry and didn't have much time to prepare it. Her wedding dress was borrowed to her by Princess Mary, wife of her cousin and her husbands first cousin, Prince Vsevolod of Russia, who wore the same dress at her wedding. Alexandra opted not to wear a tiara at her wedding as didn't want to look taller than her husband, as she already was taller than him.
Sorry for bigger post, but those are some kind of interesting details regarding jewels that I read in her book.