A mysterious 18th-century necklace featuring around 500 diamonds, some believed to be linked to Marie Antoinette, will be auctioned by Sotheby’s on November 11 in Geneva. With online bidding starting on October 25, the necklace is expected to fetch $1.8 to $2.8 million. Previously owned by the Marquesses of Anglesey, it will make its first public appearance in 50 years. Its designer remains unknown.