Kate has borrowed this bracelet more than once from Queen Elizabeth.
It was gifted to QEII from Prince Phillip when they were married in 1947.
Prince Phillip dismantled his mother’s tiara (she was Princess Andrew of Greece and Denmark) and used the diamonds (set in platinum) to make a new bracelet. It certainly has a Grecian influence: the motif is primarily geometric and angular.
Queen Elizabeth II has worn it for portraits and other significant events.
Kate has worn it twice (as far as we know).
To the state banquet in October 2015.
And to the December 2015 diplomatic reception.