It is always fun to speculate on how this occurs.

I strongly suspect that the initiative comes directly from the Queen as they are her personal jewels, some of which she still wears regularly. But whether HM has let it be known that specified jewels are "available" or whether HM makes an offer after she hears what a royal lady plans to wear on a specific occasion, is less easy to gauge. I think the latter is more likely. I do, however, suspect that some items which HM has lent to the Duchess of Cambridge might be "permanent loans" -- for example the diamond earrings which the Duchess wore so frequently in the early years of her marriage.



I think it is safe to say that some items, such as the Greville tiara, have been given to the Duchess of Cornwall as "permanent" loans.