Which "Queen Elizabeth" (Queen Elizabeth II or Queen Elizabeth the Queen Mother?) are you refering to? And which "collet necklace" (there are quite a lot in the British royal collection...)?
Queen (Mother) Elizabeth's collet necklace - called "Queen Elizabeth's Coronation Necklace" in Hugh Roberts' book The Queen's Diamonds - was never worn by Queen Elizabeth II, and was probably not worn in Wales by the Queen Mother in 2002, as she died in March that year.