From time to time Queen Elizabeth the Queen Mother wore a distinctive gold and diamond flower brooch, usually to horse racing events. Until recently I assumed that this was a brooch she acquired sometime in the last quarter of the twentieth-century.
But I was mistaken, as this photo from October 1946 shows. On this occasion QEQM with the then Princess Elizabeth and Princess Margaret went on a short sea trial of RMS Queen Elizabeth prior to its first post war peace time voyage. HM had originally launched the liner for Cunard in 1938 and apparently had remarked that she would like to do a sea trip in it.
There is no doubt that it is the same brooch as the one QEQM occasionally wore in the latter years of the 20th century but, from time to time, HM would pin it at a slightly different angle.
QEQM began to wear numerous new jewels almost immediately after WWII, this brooch being one of those jewels. Because of the timing it is possible that the brooch may have been one of Mrs Greville's jewels.
A stronger possibility, in my opinion, is that this brooch was given to QEQM by Cunard either as a gift when she originally launched the liner in 1938 or as a gift when she attended the trials of the refurbished liner prior to its first post war peace time voyage in 1946. The press reported in October 1946 that QEQM, the then Princess Elizabeth and Princess Margaret were all given "souvenirs" of their role in this trial. QEQM was known to immediately pin on a brooch gifted to her which makes it possible that this was a 1946 gift, but I didn't see any such press reports but, as any presentation was likely to have taken place privately, this may not be significant.
Getty has a video showing HM during the 1946 trial. In the gloom of a foggy day, the sparkle of the diamonds is impressive.
https://www.gettyimages.com.au/detail/video/queen-elizabeth-and-princesses-elizabeth-and-margaret-news-footage/640474214