Best of Birthstones: Pristine Pearls

In 1917, Pierre Cartier famously traded a double-stranded, natural pearl necklace, valued at the time for $1 million, and $100 in cash for a mansion on Fifth Avenue, where he established Cartier New York. Pearls have always exerted a powerful allure. Worn by royalty, legendary beauties and style icons – just think of Audrey Hepburn's pearl-and-diamond statement necklace in Breakfast at Tiffany's or Coco Chanel's signature layered strands, still a staple in the fashion house's runway collections today. Unlike diamonds, natural pearls emerge as finished gems, and the finest are lustrously silky and perfectly round, which makes finding a match an incredible feat. Click ahead for 18 pearl jewels coming to auction in London, Geneva and online–Stephanie Sporn