This brooch takes the form of a maple leaf, the emblem of Canada. It features baguette diamonds to create the stem and veins of the leaves.
Order of Splendour says that The Queen Mother, then Queen Elizabeth, received the maple leaf brooch from George VI before the couple toured Canada in 1939, while Leslie Field’s implies it was a gift during the tour (‘Her Majesty was presented with a large diamond encrusted maple leaf brooch’).
She then leant it to her daughter in 1951 for the same reason, and The Queen has lent it to The Duchess of Cornwall and Duchess of Cambridge on their first trips to Canada.