The diamond went for well above the expected $15m and more than triple the price paid for it at auction almost two decades ago. The 76.02-carat diamond, with perfect colour and flawless clarity, came from India.
The seller, Alfredo J. Molina, of California-based jeweller Black, Starr & Frost, said the winning bidder is going to donate the diamond for display.
Named for Archduke Joseph August, the great-grandson of both a Holy Roman emperor and a French king, the diamond passed to his son, Archduke Joseph Francis, who put it in a bank vault.
It surfaced at a London auction in 1961, then at a Geneva auction in 1993, when Christie's sold it for $6.5m.