The “Williamson Pink Star,” an 11.15-carat fancy vivid pink diamond, sold for more than $57 million at Sotheby’s Friday, exceeding expectations and shattering an auction record.  A private client from Boca Raton, Florida, bought the 11.15-carat internally flawless cushion-cut diamond, paying 453.2 million Hong Kong dollars ($57.7 million), well above its $21 million pre-sale estimate.  The buyer, who was not named by Sotheby’s, renamed the stone “The Rosenberg Williamson Pink Star.”  Mined at the Williamson mine in Tanzania and cut by Diacore, the Williamson Pink Star is the second largest IF fancy vivid pink diamond ever to appear at auction.