This is also known as Queen Mary's choker necklace, as it was given to Queen Mary in 1911 to wear at the Delhi Durbar.
It was inherited by Queen Elizabeth II but she never wore it, as she didn't like chokers. But she did lend it to Princess Diana to wear when she was married to the then Prince Charles.
King Charles III inherited it on his mother's death and lent it to Catherine, Princess of Wales:
The choker was a gift from the Maharani of Patiala. In the 1920's the setting of the diamonds and 16 cabochon emeralds was modified to an Art Deco style, and it's remained like that ever since. Here is Princess Diana wearing it: