I believe that the first photo posted by Kongetro shows Queen Mary wearing the emerald and diamond necklace presented to her by the Ladies of India during the Delhi Durbar in 1911.
Queen Mary wore it to the Durbar on 2 separate occasions.
And (here it is worn with the rest of the emerald parure but without the emerald drop)
This is the necklace which Queen Mary later had dismantled and reconfigured as the choker, which was later worn by Princess Diana.
Re dating the photo posted by Kongetro.
Various photo agencies have either the same or photos from the same photographic session with dates ranging from circa 1910 to circa 1914.
The
BnF has a copy of one and gives a very precise date: 25 June 1913.
Below is a detail from one of the photos in the BnF
From 24 June to 26 June 1913 President Poincare paid a visit to London. Queen Mary does not feature in his welcome and drive through the city as Mme Poincare did not come to London. However, Queen Mary did attend day time ceremonial events including a visit to watch a horse show at Olympia. I strongly suspect that the Queen may have worn this outfit and the Ladies of India necklace for that occasion but, unfortunately, the only images I could find are too indistinct to say that with certainty.
The President and the Royals at Olympia
Comparison photos of the necklace worn at the Durbar functions and the 1913 photos.
I could not find a copy of the photo used by Kongetro in the UK press, or in Royal Collection or the NPG UK.
I wonder if anyone would have the time to look at what is in the French press?