According to Roberts in The Queen's Diamonds, page 272 note 1, the Princess' exclamation of delight was heard when she saw the gift of 5 large diamonds presented after she opened the Graving Dock on 3 March 1947.
--Previous Message-- : This from Fields book I don’t know how : accurate it is but here it goes. : Pss Elizabeth was presented by Field-Marshal : Smuts with her gift from the Government: a : silver casket containing a long chain : necklace of 21 large diamonds that had be : cut and polished in Johannesburg, the : largest of which was 10 carats. Forming : connecting links between each of the 21 main : stones were two small brilliant-cut diamonds : mounted on either side of a baguette : diamond. The casket was handed to Pss : Elizabeth as she stood by a microphone and : the entire nation heard her gasp of delight : when she saw the necklace. From that night : on, she always called them ‘my best : diamonds’. : PJ : : --Previous Message-- : : : …from The Queen's Diamonds : : The gift from the Government of the Union of : South Africa consisted of a long chain of 21 : graduated brilliants - the largest of 10 : carats - each separated by a baguette and : two small brilliants." : The necklace was presented in this : configuation on 21 April 1947. : : The detachable snap-piece was added later : using a 6 carat stone given to her by the : Chariman of De Beers on 18 April. : : The necklace was exhibited in this form with : her wedding presents. : : In my earlier pic the Queen wears the : necklace on a visit to Paris in May 1948. : : Five years later the necklace was shortened : to 15 large stones and a bracelet made from : the six removed and the snap-piece. : : Edit: Footnote - According to Charlotte : Gere and John Culme with William Summers in : "Garrard The Crown Jewellers for 150 : Years 1843 1993", : the 21 diamonds were presented to Princess : Elizabeth unset, then Garrard made the : necklace in London. : : : : : : :