Bunny Mellon’s 9.51-Carat Fancy Blue Diamond Is Heading to Auction

Bunny Mellon knew a thing or two about breathtaking gardens—and, it seems, stunning gemstones.

The American horticulturalist’s former fancy vivid blue diamond, aptly named the Mellon Blue, is heading to auction via Christie’s Geneva’s Magnificent Jewels event. The 9.51-carat gem comes with a sky-high estimate to match its beauty: It’s expected to fetch a whopping $20 million to $30 million when it pops up on the block this November.

The Mellon Blue is graded…