Loyalty Has Its RewardsMarc Johnson knows about loyalty. It has helped him build his 3½-year-old custom design business, Cove Jewellery, in Andover, U.K., into a lean, mean one-man operation that thrives on commissions, remodels, and repairs (Johnson alone works the front and back of the house). With 100 percent of all jewels made in-store, Johnson makes a living in a way many jewelers only dream about: “I have no commercial product lines at all, just our stock jewelry display pieces,” he says.Business accelerated in fall 2010, and Johnson, shortly after launching a website that touted his talents, decided to thank his best customers with his own frequent-buyer program. Specifically, he instituted the Platinum Loyalty Card for “good customers who love jewelry,” he says. Marc Johnson, the owner and lone metalsmith at Cove JewelleryThe plan works like this: When customers refer pe