The question of ownership is a very fascinating and complicated one.
What is or is not part of the Royal Collection, for example?
It appears to be somewhat arbitrary. For example, if you do random searches on the RC website, you'll find stuff one would expect could be private property (things like recent family photographs, for example, or small nicknacks). But searches on jewelry, tiara and similar terms only returns "old" stuff and things like the Regal Circlet (which is an heirloom of the Crown).
For example, Queen Alexandra's little Faberge animals vs her jewels. It is well documented that QA went to the Faberge store and bought the little animals with her own money (I'm intentionally simplifying here). This is fairly clear indication to me that they were QA's private property, yet they are listed as part of the RC. On the other hand, the Cartier collier resille for which there is a receipt in Cartier's archives showing it was sold to Queen Alexandra (a similar indication that it was QA's private property), is not. (A search on "Cartier necklace" only returns two items; a photograph of Queen Mary wearing the collier resille, and toy jewelry given to young Princess Elizabeth).
Who decides what belongs to the RC and what doesn't, and what are the criteria? Interesting.....