Open this photo in gallery: An employee of Christie's auction house holds the 'Briolette of India' diamond necklace, on May 8.FABRICE COFFRINI/AFP/Getty Images The 700-piece private jewellery collection of the late Austrian billionaire Heidi Horten, the world’s largest and most valuable collection of its kind to come to auction, could sell for more than $150-million, Christie’s said on Monday. Some of its most spectacular pieces include the 90-carat “Briolette of India” diamond necklace by Harry Winston, and the “Sunrise Ruby” and diamond ring by Cartier that is worth up to an estimated $20-million. The collection, acquired between the early 1970s...