Thank you Franck. So far I have not found another photo from this event. I was hoping for one which might give a better view of the brooch.

Do you know the year? That might help to narrow down the search. I know that QEQM wore long dresses like this to a few formal garden party events during the north American tour as well as in the UK, so a place name would also narrow the search if you have any additional information.