The photo which I initially posted in this thread showed QEQM at a Garden Party at Bulawayo during the South African tour of 1947.
Apart from HM's pearls, I was interested in her brooch.
As my post immediately above would suggest, I don't think that the brooch is the one Franck has been trying to identify.
I think that Bryce was correct when he mentioned that it might have been a badge.
I haven't had time to do a newspaper search to see if the King and Queen inspected troops and, if so, which ones, but it seems highly likely. That was one the many things which they did in South Africa, and also visiting and acknowledging those who fought in WWII.
I wonder if QEQM might have been wearing her Black Watch badge and that the top diamond portion has faded into the background of HM's dress? I think I can make out something at the bottom of the jewel.
Other images of HM wearing the Black Watch badge.