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 store. I would recommend it to anyone!"Her reaction may account for the uncommon but pragmatic policy of Gallery of Jewels, a San Francisco mainstay with locations on Union Square, 24th Street, and Fillmore Street. Employees greet shoppers who come in toting loose "discount" diamonds with: "That's exc