Queen Mary's Garter Insignia was the gift of the Empire Marys' to the Queen upon the occasion of her coronation.
Over 14,000 pounds was raised and of that 10,000 was allocated to a charity of the Queen's choice. The remainder was used to purchase and present personal gifts to Queen Mary. She received two portraits (of the King and the Prince of Wales) and of greater interest to us, her Garter insignia.
It was created by Garrards and consisted of four items: the Garter, the Badge, the Star and the Shoulder Brooch.
The brooch is described in contemporary reports as consisting ". . . of a double row of fine brilliants, five in each row, is used to secure the ribbon to the dress." A picture of it along with the other items was taken while the insignia was at Garrards and the picture appeared in the Illustrated London News and newspapers such as the Brisbane Courier.
Boffer had thought that the 14 stone brooch had likely been commissioned as part of Queen Mary's Garter insignia but that is not accurate.
I personally believe that the Queen often wears her grandmother's 10 stone Garter Insignia shoulder brooch, which is almost impossible to distinguish from Queen Victoria's bar brooch.
Queen Mary wrote in her thank you letter: "The beautiful insignia of the Garter...will be treasured by me throughout my life, and handed on as precious heirlooms to those who come after."
Queen Mary kept her Garter Insignia until her death and then it is logical to assume that it was passed on to Queen Elizabeth II.