On October 7, two extraordinary diamonds will hit the auction block: An enormous 118.28-carat D-flawless diamond nearly the size of a small chicken egg—the largest D-flawless diamond ever to appear at auction—and the 7.59-carat internally flawless vivid Premier Blue Diamond, the largest round vivid-blue diamond to be sold at an auction. Both sales are expected to set record prices at the Sotheby’s Hong Kong auction.
The white diamond—which was discovered in 2011 as a 299-carat piece of rough in a southern Afr…