It seems to be generally accepted that the Londonderry emeralds were depicted in this painting of Edith, Marchioness of Londonderry by Laszlo.
I accepted the possibility of that, until I took a closer look at the painting. I feel that the artist has depicted either amethysts or perhaps rubies; definitely not emeralds.
What do others think?
I think that Edith, the 7th Marchioness was wearing the suite when photographed in 1939
Even though this shows a tiara, not a necklace, the elements seem to match those seen in later photos of the necklace, although the earrings are different.
Slightly closer view
Edith, Marchioness of Londonderry was said to be wearing emeralds when the photo below was taken in 1930, but her arm is obscuring a clear view of her necklace.
Elizabeth, Lady Sudeley is said to be wearing the Londonderry emeralds in this photo.