I am not sure if this diamond brooch has been discussed previously.
On page 37 of The Queen's jewels Field described a brooch which was inherited by the present Duchess as 'an oblong diamond brooch with a square cluster centre and link ends' which had been given to Princess Marina as a wedding present by Queen Mary. Field went on to say that Princess Marina had pinned it to her wedding dress. I found the best glimpse of it on a video.
Princess Marina also appears to have worn it on her hat as she left for the honeymoon.
Below is the present Duchess wearing it in 1969.
Numerous jewels can be seen in the video. The brooch can be seen on the wedding dress from 7.50 minutes and on her hat at approx 8.34 minutes onwards. Some of Princess Marina's bracelets are seen at approx 11.40 minutes onwards. From 13.17 minutes more royal jewels can be seen and at 14 minutes there are some interesting views of the Duchess of Gloucester wearing her pearl and emerald necklace. At 14.30 we can see more jewels worn for the christening of Princess Alexandra and at 16.55 minutes we get to see the pearl and diamond snake tiara the Princess was given as a wedding present by her mother.