White diamonds are the world’s most expensive gems.White diamonds are the world’s most expensive gems. The ideal stone is like a piece of ice, whiter than white, graded ‘D’, the purest possible grading, and cut with exquisite precision. Only a handful exist. Ten years ago a pure white, pear-shaped 100.10 carat diamond (pictured here), classified as ‘D’ and internally flawless, was sold by Sotheby’s in Geneva for $16,548,750. Named ‘The Star of the Season’, it is still the most expensive precious stone ever sold at auction. Today its whereabouts are a closely guarded secret, but it is likely that it sits contentedly, if a little unloved, in a secure bank vault, handled on rare occasions by white-gloved admirers.