I agree that I have engaged in speculation, Nellie - but only to the extent that I thought, after having considered the evidence available to me, it seemed reasonable to deduce that Queen Mary might have purchased several jewels at once, and the date, based on the available evidence, might be 1919. Obviously, based on information from the Royal Collection site, Roberts and photographic evidence, Queen Mary started to acquire some jewels as early as 1919 and had others in her possession by 1923.
Nevertheless, I would agree that her brother could have sold items piece by piece to Queen Mary; however, if he needed money, financially it would have made sense to sell a quantity of jewels at one time in order to have money to invest, rather than selling individual items to pay for current bills or for providing money for living expenses for a short period.
I acknowledge that, unless historians have access to the Teck/Cambridge papers and financial records, obviously we cannot test that scenario. What we do have information about from Roberts and the Royal Collection site is that Queen Mary acquired by purchase the bracelet with Queen Adelaide's portrait in 1919. By 1923 Queen Mary was photographed wearing the brooch which QEII has now turned into earrings and, according to Roberts, the Duchess of Teck's flower brooch, which was given to QEQM by George VI, was in Queen Mary's possession by 1923, the year in which it was given to QEQM.
So, yes I am again speculating, but it is in the knowledge that the Teck family was not wealthy, and Queen Mary's brothers did not receive any income from the government of the UK to support their living expenses.
Most of us on this board, can normally judge the accuracy of information (and our deductions) regarding English royal family jewels only on the following: published items based on privileged access to sources (such as Roberts' book), the information published on the Royal Collection site, and photographic evidence. Regarding the latter, we can only judge if dates given are reliable by comparing dates given in various publications and websites. As we all know there are discrepancies in Roberts and in the Royal Collection website, as well as sites such as Getty images etc. I won't bother mentioning the numerous other publications which purport to have definitive knowledge, but which have been proved to be wrong time after time.
To sum up. Yes, I speculated but, with the knowledge we have available, it seemed reasonable and not farfetched.
I would like to draw posters' attention to the fact that Roberts does not give any dates, or indeed definite proof, that Queen Mary purchased the Teck tiara or the hoop/circles necklace which were worn by QEQM. He says only that the tiara was probably given to QEQM by Queen Mary.
Why this is so may be explained by two things. Probably neither jewel was photographed in Queen Mary's jewel books, and secondly Roberts did not have access to Queen Mary's correspondence or, more importantly, her financial records which might show a payment to her brother.
That is another supposition by me - but I think one which may have some validity given what Roberts says.
Posters have previously assumed in this thread that the Teck tiara and necklace were owned, and then lent, by Queen Mary to the Marquess of Cambridge.