The package arrived—signed, sealed, delivered—right on time. There was one hitch: It was empty. A $30,000 diamond had vanished without a trace.Avi Jacobowitz, the New York dealer who sent the stone, notified the shipping service and collected insurance for the lost item. But he feels he could have made more money if he sold it. To this day, no one has found the stone—or its new, unlawful owner. “It was an excellent stone; it was a pity to lose it,” says Jacobowitz. “It’s a violation knowing someone stole something from you. It’s not a good feeling.”Yet it’s a common one, industry experts say. Shipping losses are “a huge problem,” says John Kennedy of the Jewelers’ Security Alliance. “You have a situation where it’s normal for a person in the trade to have a couple of thousand-dollar losses a year.” Last year, Jewelers Mutual Insurance Co. paid off 400 clai