The Princess of Wales wore the Prince of Wales Feathers Pendant, often donned by the late Diana, Princess of Wales, for her first outing at a state visit in her new role.
Kate wore the sparkling diamond pendant as a brooch on Tuesday, with its detachable cabochon emerald at the bottom, on her regal purple Emilia Wickstead dress which she matched with a Sean Barrett hat.
The pendant is made of 18 round cut diamonds with small emeralds around the three Prince of Wales ostrich feathers, a crown and the motto "Ich Dien" (I Serve).
It was originally a wedding gift to Princess Alexandra of Denmark from the "Ladies of North Wales" when she married the Prince of Wales, later King Edward VII, in 1863.
It was later given to the Queen Mother and then Diana when she married Charles, then the Prince of Wales, in 1981.
Kate became the Princess of Wales when the King announced his eldest son William as the Prince of Wales the day after the death of the Queen on 8 September.
Both Kate and William greeted South African president Cyril Ramaphosa and his wife on Tuesday morning on behalf of the King, before escorting him to Horse Guards Parade for the pomp and ceremony of the official welcome.
King Charles then hosted his first state visit since becoming British monarch as he welcomed the president to Buckingham Palace.
The 74-year-old monarch rolled out the traditional pomp and ceremony for the first time as head of state, as Britain seeks to bolster its relations with its biggest trading partner in Africa.
Mr Ramaphosa's two-day trip marks the first state visit to the UK by a world leader since that of former US president Donald Trump and his wife Melania in 2019.