The emerald necklace of Consuelo Yznag Montagu, the Duchess of Manchester and friend of Queen Alexandra was placed up for auction by Sothebys in 2015. It didn't sell.

https://www.sothebys.com/en/auctions/ecatalogue/2015/magnificent-jewels-n09440/lot.349.html

It now appears in the up coming Magnificent Jewels and Noble Jewels in Geneva later this month. It is not yet currently listed on the website but can be seen at page 58 of the PDF auction catalogue.

https://www.sothebys.com/en/buy/auction/2020/magnificent-jewels-and-noble-jewels-part-ii

Ursula has a page on it

https://royal-magazin.de/england/manchester/manchester-duchess-emerald-jewels-necklace.htm