The 10-Carat Mediterranean Blue Diamond Could Fetch $20M at Sotheby’s

Sotheby’s next jewelry auction wants to help you enter your blue period.

To be sure, the auction house’s High Jewelry Sale, taking place on Tuesday, May 13 in Geneva, will offer up a slew of gems certain to entice collectors—but there is one definitive headliner: the Mediterranean Blue, a 10.03-carat, fancy vivid blue stone. Expected to hammer down for $20 million (CHF 18 million), the ultra-rare gem has a Type IIb designation, a classification that includes less…