You can thank the wobbly American economy for the explosion of silver jewelry designers in recent years. And now that the category is a fixture in fine stores nationwide, sorting through the myriad options can be daunting. JCK has cut through the clutter by highlighting three silver brands with great potential for American retailers: One company offers a clean, modern look; another goes for a darker, decidedly edgier effect; and a third celebrates a romantic, bohemian style.Haverhill Inc.Her line debuted in March, but Haverhill Leach of Haverhill Inc. in Providence, R.I., has long-standing tie