In a recent Facebook discussion, ­jewelers weighed in on a delicate situation: What to do when a customer brings in a diamond bought off the Internet? Store owners from the East Coast and Midwest grumbled about the trend, calling it "tacky," among other things.But Jennifer Kroeger, owner of Relic Charm in St. Paul, Minn., reminded everyone why the practice is thriving. Three years ago, when she was a sales associate at Jared the Galleria of Jewelry in Maplewood, Minn., she opted to buy a loose diamond online: "The diamond was a third of the price, even with my discount working at a jewelry st