Hello fellow forum followers: I'm new to the forums, but not new to royalty-watching.
IMHO, it seems that, rather than being stingy as has been suggested, The Queen is very much a traditionalist at heart, especially in matters of royal prerogatives.
Since the queens who preceded her back to Victoria kept the jewelry collection together until it was time to distribute after their deaths, AND since both Victoria and Mary made distributions privately and to the Crown collection, I suspect that QE II will do the same.
And the most important jewels that she has loaned have primarily been to the wife of her next in line: first Diana was given some good jewelry, which were loans, and now Camilla seems to have the Delhi Durbar at her disposal which MUST be in the crown collection, and she has worn the Boucheron tiara AND the fabulous Greville five-string diamond necklace given to QE the QM and left by her to QE II.
Certainly no one else in the Royal Family has worn the truly important gems other than the Queen, the Princess of Wales and the Duchess of Cornwall. I certainly doubt that it is due to the Queen's personal preference or to stingyness, but most likely has to do with the appropriateness of having the truly important jewels being worn outside of the direct line of succession.
Weighing further on this topic, unless the Middletons buy her a tiara, Catherine will no doubt be loaned something good for her wedding, in recognition that she is to be the wife of the heir's direct heir. If my logic is correct, and tiara-wearing has symbolism and connotes a quiet statement made by the Queen, the tiara loaned to Catherine will likely not be one that has been worn outside the direct line of succession (or the consort thereof). The one exception to this was that, for her wedding, the Pr. Royal wore the fringe tiara that her mother wore for her own wedding.
I found it interesting that for Margaret, Sarah and Sophie, NEW tiaras were bought/made, despite all of those in the queen's possession... There must, therefore, be some special tradition or consideration attached to those in the Crown collection and those that were bequeathed personally to The Queen. Otherwise, wouldn't it have been more like the typically frugal Queen to have just reached into her collection for these pieces?
If they are eventually sold (like Margaret's Poltimore), there will be no royal tradition to be flouted, as there was when some of Victoria's pieces were sold by other heirs. The Queen has added little to the collection (other than state gifts), but I suspect that she will choose to keep the bulk of her personal collection together for direct heirs to the throne, other than small gifts to her other grandchildren, nieces and nephews, most of whom will never have the need to wear a tiara. Perhaps Harry's wife will get a tiara, and some good pieces, for he, like the Princess Royal in her generation, will probably remain in active royal duty.
Again, just one royal watcher's opinion!
..i wanna see more jools of darling Diana around her divine neck and wrists..i'm gonna die if she wears Di's wedding dress n' shoes n' that absolutely glorious veil 
..no kiddin, i'll be in awe for the rest of my life! 
