The world’s largest blue diamond, “The Blue” has internal flawless clarity. Its size and quality have caught the attention of industry insiders, who believe it could sell for around $25 million at next month’s Christie’s Magnificent Jewels event.

The auction will take place May 14 at the Four Seasons Hotel des Bergues in Geneva, where the 13.22-carat rock will be set between two other pear-shaped diamonds – 1.00 and 0.96 carats respectively.

Describing Lot 260, Christie’s said it is the “largest fancy vivid blue diamond in the world”. But the auction house did not reveal the diamond’s owner.

The fancy ring is accompanied at the auction by a rare light pink square-cut diamond. The 76.51-carat jewel and its necklace are due to fetch around $10 million.

But expectations are not always much to go by. When the hammer fell on “The Orange”, the world’s largest orange diamond, last November, the final bid was $35.5 million – way above the estimated $17-20 million value.