As I noted below, I don't have a great deal on the jewels of the Duchesses of Atholl.
In the late 19th century and early part of the 20th century, two of the Duchesses wore large emeralds which were said to be Atholl family jewels. Unfortunately, I don't have any high resolution of these jewels.
Some press reports about the emeralds.
1907. At the time there was no Duchess so the wife of the heir, Lady Tullibarde, wore them.
1900
1900. Says emeralds are square.
1922 says has an emerald tiara
1923
Some images which I think, based on the descriptions above, show the emeralds.
1952 Necklace
1949 necklace
1935 tiara. The Duchess is second from the right as we look at the photo.
1938 tiara
The necklace in this photo from 1935 has me interested. Although the photo is indistinct, I suspect that it may be the necklace worn by Sara Troughton in the image 992234177 posted below.
Detail of Sarah Troughton from 992234177's image.
Necklaces compared
Another thought which occurred to me is that perhaps the tiara worn by Sarah Troughton is the grand emerald necklace with the emeralds removed.
What a shame I don't have higher resolution images of the grand emerald necklace!