In the market for a 49-carat diamond necklace? When you’ve really messed up, head here to shop three floors of custom, designer and estate jewelry and watches that are sure to get you off the hook. The old-school grandeur of the mahogany-lined Newbury Street location isn’t an act; Shreve, Crump & Low is billed as America’s oldest jeweler, having first opened its doors in 1796. And after more than two centuries operating solely in the Bay State, the storied jeweler opened its first shop in Connecticut in December.