The Williamson Diamond is considered the finest pink diamond ever discovered. It was found in October 1947 at the Mwadui mine in Tanganyika, owned by the Canadian geologist and royalist Dr John Thorburn Williamson (1907–58), after whom the diamond was named.
The uncut stone, weighing 54.5 metric carats, was presented by Dr Williamson, as a wedding present to Princess Elizabeth in 1947.
The stone was cut by the firm of Briefel and Lemer of Clerkenwell into a 23.6-carat round brilliant, maximising its luxurious rose colour.
Queen Mary and Princess Elizabeth inspected the work of cutting and polishing during a visit to the Clerkenwell premises on 10 March 1948. When the Queen acceded to the throne in 1952, there was speculation that the stone might be mounted for use at the coronation. However, it was instead set as the centre of a brooch in the form of a jonquil flower, designed by Frederick Mew of Cartier, in 1953.
Dr Williamson wished to add further pink diamonds to his original gift, but as these were not available, he gave The Queen 170 small brilliant-cut diamonds, 12 baguette-cut diamonds and 21 marquise diamonds, which were used to form the petals, stalk and the leaves of this brooch.