The world's most expensive green gemstone: a rare emerald was sold at auction for $9 million.

A rare square emerald weighing 37 carats set in a diamond frame was sold at an auction in Geneva for 7.8 million Swiss francs. This unique piece of jewelry belonged to Prince Aga Khan, a socialite, descendant of the Prophet Muhammad, and leader of the Nizari Muslims who worked with the UN. The gem was created for British socialite Nina Dyer, to whom Aga Khan was briefly married. This is reported by Daily Mail.

In 1960, Prince Sadruddin Aga Khan commissioned Cartier to set the Colombian emerald into a brooch adorned with 20 marquise-cut diamonds for British socialite Nina Dyer, whom he was married to for a short time.

Dyer converted to Islam and took the name Shirin. They had no children and divorced in 1962. In 1969, Nina put the emerald up for auction to raise funds for animal welfare.

Coincidentally, this took place at the very first Christie's auction of its kind in Switzerland on the shores of Lake Geneva, and this year the emerald reappeared at the 110th auction.

It was purchased by the jewelry house Van Cleef & Arpels, and a few years later, it changed hands to American jeweler Harry Winston, known as the "King of Diamonds."

"Emeralds are currently in vogue, and this one meets all the criteria. An emerald of this quality might come up for sale once every five to six years," said Max Fossett, head of Christie's jewelry department.

The previous record-setting emerald, also adorned with diamonds from the Bulgari fashion house, was sold for 6.5 million dollars at an auction in New York, where part of the famous jewelry collection of Hollywood legend Elizabeth Taylor was auctioned off.

The current auction in Geneva was notable for its collection of historical jewels with intriguing stories. For instance, a diamond necklace associated with the scandal that tarnished the reputation of Queen Marie Antoinette will be auctioned for 2.1 million pounds sterling.

This antique necklace was also worn at the coronation of King George VI and Queen Elizabeth II.

A rare piece dated around 1776 is made from 300-carat diamonds, presumably sourced from the famous Golconda mines in India. It features a scarf design with a pair of tassels.

Experts believe that the necklace may have been made from some of the diamonds involved in the diamond necklace case—a theft committed in the 1780s from the royal jewelers Bemer and Bassange by Countess Jeanne de la Motte.

She forged Antoinette's signature to obtain the necklace. Despite her innocence, the queen's reputation was never restored.