Kate complemented the necklace with a pair of drop pearl and diamond earrings from Diana’s private jewelry collection. Kate last wore the Collingwood pearl earrings, which have become a go-to favorite for her, in July at a state banquet at Buckingham Palace for King Felipe and Queen Letizia of Spain.
Kate arrived at the anniversary celebration at Windsor Castle with Princes William and Harry. They were joined by Queen Elizabeth and Prince Philip's children, grandchildren and close friends for the dinner.
William and Kate are expecting their third child in April, providing Queen Elizabeth and Prince Philip with their sixth great-grandchild.
Harry's girlfriend, U.S. actress Meghan Markle, is in London but was absent from Monday's celebration.
It would be unlikely for Markle, 36, to attend the event unless she and Harry, 33, were already engaged. Markle and Harry are expected to celebrate Thanksgiving together later this week.
Earlier Monday, Queen Elizabeth, 91, presented the Duke of Edinburgh, 96, with a special platinum wedding anniversary present.
Buckingham Palace announced she appointed Philip to be a Knight Grand Cross of the Royal Victorian Order for his services to the Sovereign.
Queen Elizabeth and Prince Philip's anniversary celebration was also marked publicly with the ringing of the bells of Westminster Abbey.
The 21-year-old Princess Elizabeth married then-Lt. Philip Mountbatten at the Abbey Nov. 20, 1947. Five years later, Elizabeth ascended the throne after the death of her father, King George VI.