A 5.5-carat triangular-cut diamond billed as the largest fancy vivid blue-green diamond known to exist sold for more than 13.5 million Swiss francs ($17.3 million) on Wednesday, Christie’s said, calling it a record price for a stone of its kind sold at auction. The “Ocean Dream,” the standout offer at the auction house's Geneva sale of jewelry, was found in Central Africa in the 1990s. The price easily topped the presale estimate to fetch 7-10 million francs (around $9-13 million). Rahul Kadakia, president of Christie's Asia Pacific , said that an unspecified private client was the buyer, and the stone took about 20 minutes to sell — an indication that interest was high. The price was more than double that of the roughly $8.5 million that the gem, which was featured among rare colored diamonds at the Smithsonian Splendour of Diamonds Exhibition in 2003, sold for at Christie's in 2014.…