Princess Alexandra wore the necklace (as in Bryce's photos) for a dinner at Government House, Canberra in 1959.
The Australian Women's Weekly 26 August 1959 had a black and white photo and, crucially, a description of the necklace and the Princess' bracelet.
Although the description with this photo mentions only rubies, sapphires and diamonds, I am still pondering what gemstones were/are in the necklace as I feel that the Point de Vue photo shows emeralds as well, and I suspect that the central drop is a cabochon emerald. On the other hand, it is highly probably that there was an official press release describing HRH's dress and jewels and so the description should be accurate.