The piece, from a private Asian collection, will go under the hammer in Geneva on November 11, with online bidding opening on the auction house's website on October 25.
The necklace, which is composed of three rows of diamonds finished with a diamond tassel at each end, will make its first public appearance in 50 years on Monday, and is expected to fetch between $1.8 and $2.8 million.
"It's a wonderful find because, normally, jewellery in the 18th century was broken up in order to be repurposed... so to have an intact piece of the Georgian period of this importance, this amount of carats... is absolutely fabulous," Andres White Correal, chairman of the Sotheby's jewelry department, told AFP.
"The jewel has passed from families to families. We can start at the early 20th century when it was part of the collection of the Marquesses of Anglesey," he added.
Members of this aristocratic family are believed to have worn the jewel twice in public: once at the 1937 coronation of King George VI and once at his daughter Queen Elizabeth II's coronation in 1953.
Beyond that, little is known of the necklace, including who designed it and for whom it was commissioned, although the auction house believes that such an impressive antique jewel could only have been created for a royal family.
It probably would have been made during the decade preceding the French Revolution, it added.
It is thought that some of the diamonds may have come from the famous necklace linked to what became the scandal of the "Affair of the Necklace", which contributed to the advent of the French Revolution and eventually Marie-Antoinette's death, said Sotheby's.
The auction house said the diamonds are likely to have been sourced "from the legendary Golconda mines in India".
The diamonds from Golconda are still considered to be the purest and most dazzling ever mined.
The necklace will be on public display in London until Wednesday before beginning a tour that will take it to Hong Kong, New York and Taiwan.
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