I realised that I have another photo showing Edith, Marchioness of Londonderry wearing this large brooch.
The photo shows the Marchioness at a Hunt Ball in November 1952. By that time Edith was the Dowager, although her daughter-in-law, Romaine, the wife of the 8th Marquess of Londonderry, had died in December 1951. Whether Edith, as Dowager, lost access to some Londonderry jewels after her son became the incumbent Marquess in 1949 is some thing I don't have any information on.
My eyes were drawn to the enormous stone in the bracelet worn by the Marchioness on her right arm. I assume it too had a pink gemstone.
Could the bracelet and brooch be part of the jewels which can be seen on the front of the skirt of Frances Anne, Marchioness of Londonderry in the portrait below? Does anyone know what the gemstones are? Pink topazes?