COMINGS AND GOINGS>>?Mark Vadon, who cofounded Blue Nile, is stepping down as chairman of the company and from its board Dec. 31. He will be succeeded as chairman by current CEO Harvey Kanter.>>?Paris jeweler Mauboussin has appointed ­Thierry Chaunu CEO for North America. Prior to joining Mauboussin, Chaunu was CEO for North and South Americas for the Italian jewelry brand Damiani.>>?Mahiar Borhanjoo will step down from one of De Beers’ most powerful positions, senior vice president of global sightholder sales, later this year, and will be replaced by Nigel Simson. Borhanjoo decided against moving with De Beers’ sales operations from London to Botswana.>>?Georg Jensen S/A named Alexander Weindling president of Georg Jensen North America and managing director for global jewelry and gemstones. >>?Thierry Andretta has been appointed CEO of Italian jew