A Christie's employee poses with The Aga Khan Emerald

Geneva (AFP) - A rare square 37-carat emerald owned by the Aga Khan fetched nearly nine million dollars at auction in Geneva on Tuesday, making it the world’s most expensive green stone.

Sold by Christie’s, the Cartier diamond and emerald brooch, which can also be worn as a pendant, dethrones a piece of jewellery made by the fashion house Bulgari, which Richard Burton gave as a wedding gift to fellow actor Elizabeth Taylor, as the most precious emerald.

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

Dyer then auctioned off the emerald to raise money for animals in 1969.

By chance that was at Christie’s very first such sale in Switzerland on the shores of Lake Geneva, with the emerald finding its way back to the 110th edition this year.

It was bought by jeweller Van Cleef & Arpels before passing a few years later into the hands of the United States’ Harry Winston, nicknamed the “King of Diamonds”.

“Emeralds are hot right now, and this one ticks all the boxes,” said Christie’s EMEA Head of Jewellery Max Fawcett.

“We might see an emerald of this quality come up for sale once every five or six years.”

Also set with diamonds, the previous record-holder fetched $6.5 million at an auction of part of Hollywood legend Elizabeth Taylor’s renowned jewellery collection in New York.