The largest diamond ever offered at auction is set to go under the hammer in Geneva today evening, with bids expected to top USD 25 million. The "sensational" 163.41-carat flawless diamond, suspended from an emerald and diamond necklace called The Art of Grisogono, will be one of the highlights at the Christie's autumn jewel auction. "To date, this is the largest D flawless diamond ever to appear for sale at auction," Christie's said when it first announced the gem would be part of its annual Magnificent Jewels sale. The auction house said it expected the "masterpiece of unparallelled beauty and exquisite workmanship" to fetch around USD 25 million (21.2 million euros), with other experts saying it could rake in double that amount. The flawless, D colour 11A type diamond was cut from a 404 carat rough rock which was discovered in February 2016 in the Lulo mine in Angola - the 27th biggest rough white diamond ever discovered. The rough was analysed in Antwerp and cut in New York, where a team of 10 diamond-cutting specialists were involved in mapping, plotting, cleaving, laser-cutting and polishing the giant rough rock into a polished, flawless diamond. A team from Swiss luxury jeweller de Grisogono then created 50 different designs around the 163.41-carat diamond, before opting for an asymmetric necklace with the stone as its centrepiece. "It took over 1,700 hours to create this unique jewel and involved a team of 14 craftsmen," Christie's said. The left side of the necklace is made up of 18 emerald- cut diamonds and the right side composed of two rows of pear- shaped emeralds. "The Art of de Grisogono is one fabulous Christmas present and wallflowers need not apply," Tobias Kormind, head of the 77 Diamonds firm that tracks the global diamond market, said in a statement. He pointed out that when rival auction house Sotheby's sold an 118.28-carat flawless D diamond in 2013, it was sold for USD 30.6 million. The stone up for sale Tuesday is meanwhile a "much larger diamond, and its title of the largest perfect stone could see it attract a winning bid of over $50 million," he suggested. Another highlight of today's auction is the "Le Grand Mazarin", a 19.07-carat light pink diamond, which was once set in the crowns of numerous French kings and emperors, including Napolean. The gem was named after Cardinal Mazarin, an Italian cardinal and diplomat, as well as a great art collector, who served as chief minister under Louis XIII and Louis XIV, and who bequeathed it and other diamonds to the Sun King in his will.