Queen Elizabeth The Queen Mother, wearing the Greville Bow BroochAt a Royal Film Performance in 1961, Queen Elizabeth The Queen Mother wore an enormous diamond bow brooch. This may have been the only time she wore it in public. It didn't surface again until the 2012 publication of The Queen's Diamonds by Hugh Roberts, where you can find it beautifully pictured in its full glory with even more information.
Video: The 1961 Royal Film PerformanceRoberts traces the brooch back to a 1900 commission from Boucheron by Mrs. Greville, who had a diamond tiara dismantled and made into another tiara (perhaps the one that would ultimately become what we know as the Greville Tiara) and what was probably this bow brooch. The large stones and large size would have fit right in with the rest of Mrs. Greville's collection, which was inherited by Queen Elizabeth in 1942.
Photos: via Getty Images, British Pathe screencaps