In Manhattan Beach, Calif., a relaxed, coastal dress code reigns—even at the tony Martin’s Jewelry. “It’s denim and diamonds and flip-flops,” says president Alex Weil of his staff’s work wear.Forget pantyhose and French cuffs, once de rigueur for jewelry salespeople. As casual Friday steadily seeps into every other weekday, the dress codes in many independent jewelry stores have gone from starched to, well, slacker.Of course, Weil’s shop inhabits one of the country’s great bastions of informality, Southern California. And geography, more than the caliber of the merchandise or a shop’s swanky decor, dictates the degree of employee dressiness.“The world is, in general, a more casual place,” says Performance Concepts president-CEO Kate Peterson. “Look at the customers. It used to be that you would dress up to buy ­jewelry. Stores are reflecting that.”And as member