Initially Brock of the Royal Jewels of the World Message Board posted a description of a brooch presented to Queen Mary in Montreal in 1901. See this post by Arthur of the RJWMB on 3 October 2012 where he quotes Brock's post.
Arthur's post had been prompted by information posted by Torkel, who had found a clear description of this brooch in The Times in May 1902. Torkel's information proved that the brooch worn by QEII was the one given to Queen Mary.
See Torkel's post https://members2.boardhost.com/royal-jewels/msg/archive/1412335929.html
In September 2016 I posted an image of Queen Mary wearing the brooch in 1925,
During that discussion Laura stated "I believe that this picture is from the opening of the Empire Exhibition in 1925. It makes complete sense for Queen Mary to be wearing a gift from a Dominion country." See Laura's post https://members2.boardhost.com/royal-jewels/msg/archive/1473344757.html
But none of us had taken any notice of the information which Getty made available with the image: that it was worn to the Opening of the Canada Building in London in July 1925. See Getty images https://www.gettyimages.com.au/detail/news-photo/july-1925-hm-king-george-v-with-the-his-consort-queen-mary-news-photo/78949744?adppopup=true
So, the wearing of this brooch was a special and direct compliment to Canada.
This video shows some of the ceremony, but sadly the imagery is not very clear.
I wonder if Queen Mary was wearing the brooch to secure her scarf on the occasion shown in the video below? There is something pinned on her shoulder, but I cannot be sure what it is.