Queen Elizabeth II loaned this brooch to Kate for the North American tour in 2011. The maple leaf is a Canadian icon, it’s image imprinted on the Canadian flag.
The brooch is a very detailed replica of a single maple leaf. It has what looks like 7 full leaves and a slim stem. I think this is made of either white gold or platinum (most likely white gold) and it is encrusted with white diamonds.
Kate wore it three times in Canada:
For Canada Day.
The Concert at Parliament Hall.
At Rotary Challenger Park.
According to Diana’s Jewels, this brooch was worn by the Queen Mother, Queen Elizabeth II, the Duchess of Cornwall and Kate. It was first given to the Queen Mother by her husband, King George VI, in 1939 (thanks to The Royal Order of Sartorial Splendor). It was then passed onto her daughter, who now loans it out to her daughter-in-law and granddaughter-in-law.