A rare fancy vivid blue diamond weighing 10.03 carats is going up for auction at Sotheby’s. “The Mediterranean Blue” will make its auction debut at Sotheby’s High Jewelry sale in Geneva on May 13, estimated to sell for about CHF 18 million ($20 million). 

The diamond is classified as Type IIb and its color grade, fancy vivid blue, is the Gemological Institute of America’s highest possible color grading for a blue diamond.

According to GIA, only 0.3 percent of all diamonds submitted to the lab annually display a color that would be described as predominately blue, and of those, only a small number are fancy vivid weighing 10 carats or more.   

“Like its namesake, the Mediterranean Blue is a symbol of nature’s eternal wonders. Its physical beauty, enhanced by the skill of a master cutter, is mesmerizing as is its blue color, so rarely seen,” GIA said. 

“The attributes of the Mediterranean Blue, weighing 10.03 carats in the most coveted fancy-color range, are absolutely exemplary for a diamond.”

As for the shape, colored diamonds require careful planning to balance the largest possible size, the best clarity, and the most vibrant face-up color, Sotheby’s said. 

The Mediterranean Blue is a cushion modified brilliant-cut diamond, the result of a six-month long planning and cutting process.