A spectacular and rare orange diamond, the largest known gem of its kind, was on Tuesday auctioned off for a record $31.5 million in Geneva. "At the back of the hall, 29 million francs ($31.5 million, 23 million euros). Sold!" the Christie's auctioneer said as the fiery almond-shaped gem was snapped up in a room of about 200 people in a luxury Geneva hotel.