I think it is the Queen's South African diamond necklace in its original length, as it seems to me that there is a space between each of the larger diamonds.
I wonder what the Queen Mother was wearing?? Three rows of diamonds or could it even be the diamond festoon necklace? I recollect that Arthur once posted a photo which suggested that HM was wearing that necklace even though Roberts says that George VI had the necklace set for QEII