Laurence Graff, the founder of London’s Graff Diamond, acquired one of the largest fancy vivid yellow diamonds ever discovered, a 299-carat rough yellow diamond from the Letseng mine in Lesotho. Now the gem has been meticulously cut into nine different stones: six pear-shapped, two brilliant-cut round,and the cushion-cut Golden Empress, the largest at 132 carats.…