Earlier this week, a 5.5-carat triangular-cut diamond known as Ocean Dream sold for over $17 million during Christie’s Magnificent Jewels auction in Zurich. The stone, characterized by its striking blue-green color, was set into an 18-karat white gold band and surrounded by pink and white diamonds.

The sale took just 20 minutes with the buyer remaining anonymous. Max Fawcett, Global Head of Christie’s Jewellery, highlighted that this is the second time Ocean Dream has been offered at auction; it sold for nearly $10 million in 2014 at Christie’s Geneva.

Described by the Smithsonian as one of the eight rarest diamonds in the world, Ocean Dream was part of a 2003 exhibition “Splendor of Diamonds” at the National Museum of Natural History in Washington D.C. The GIA confirmed that it is the largest Fancy Vivid blue-green diamond of natural color they have examined.

Though naturally occurring colored diamonds exist in various colors, with reds and purples being the most rare and yellows the least, Ocean Dream stands out for its unique combination of tone and size. It was extracted from central Africa in the 1990s and weighed nearly 12 carats uncut.

The May 13 sale featured a total of 87 lots at Zurich’s Four Seasons Hotel des Bergues, bringing in $66 million in total. Pieces by Cartier and Van Cleef & Arpels were also included.