The Apollo Blue, estimated to be worth about $38 million to $50 million, weighs 14.54 carats and has been mounted as an earring. According to Sotheby’s (BID), it belongs to an elite group of diamonds (comprising less than 0.5% of all diamonds) which are pure with the presence of boron, giving it its unusual blue color. “Diamonds of this quality, with this extraordinary depth and sophistication of colour, and of this size are unfathomably rare,” the auctioneer’s website says.

Another diamond earring, the cherry-blossom Artemis Pink, estimated at $12.5 million to $18 million, is also on sale Tuesday. Although the diamonds together make up what is thought to be the most expensive pair of earrings in the world, Sotheby’s is offering the jewels separately “on account of their extreme rarity, power and presence.”

David Bennett, Worldwide Chairman of Sotheby’s International Jewellery Division, described the diamonds in a statement as “by far the most important pair of earrings ever offered at auction.” They are being auctioned in Geneva as part of the auctioneer’s Magnificent Jewels & Noble Jewels sale. Other jewels on auction include a pair of emerald, onyx and diamond earrings by Cartier and a pair of fine diamond ear clips by Harry Winston.