The Hanover Russian Fringe Tiara
This tiara (which can also be worn as a necklace) was made for Queen Mary in 1919. It is not, as has sometimes been claimed, made with diamonds that had belonged to George III but re-uses diamonds taken from a necklace/tiara purchased by Queen Victoria from Collingwood & Co as a wedding present for Queen Mary in 1893. In August 1936 Queen Mary gave the tiara to Queen Elizabeth.
Queen Elizabeth later loaned it to her daughter, Princess Elizabeth, the future Elizabeth II as "something borrowed" for her wedding in 1947.
As The Princess Elizabeth was getting dressed at Buckingham Palace the tiara snapped. Luckily the court jeweller was standing by in case of emergency.
The jeweller was rushed to his work room by a police escort. Queen Elizabeth reassured her daughter that it would be fixed in time, and it was. In the above picture you can see where the tiara detached.
The Queen Mother later also loaned it to her granddaughter The Princess Anne for her marriage to Captain Mark Phillips in 1973.