The sparkling diamond pendant, which can be worn as a brooch as Kate did on Tuesday, 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 is the first time Kate has worn the jewellery, and she chose to do so with its a detachable cabochon emerald at the bottom.
Dressed in a regal purple Emilia Wickstead dress and a matching Sean Barrett hat, Kate joined the Prince of Wales as they played a key part in the South African state visit in their new roles.
Heir to the throne William and the princess met the South African president Cyril Ramaphosa on Tuesday morning on behalf of the King and escorted him to Horse Guards Parade for the pomp and ceremony of the official welcome.
The Prince of Wales Feathers Pendant was 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 the then Prince of Wales in 1981.
Kate became the Princess of Wales when the King made his eldest son William the Prince of Wales the day after the death of the late Queen in September.