How do jewelry thieves know if a pearl is valuable enough to be stolen?

There may be more than one way, but Arthur Barry, a top-class thief in the 1920s, who managed to steal an average of half a million dollars a year, has a unique and fast technique. In the theft at the Plaza Hotel in 1925, Barry managed to bring jewelry worth $ 750,000 owned by James P. Donahue. Barry silently picked up the jewelry from the dresser, while Donahue was soaking in the bath a few yards away. But the jewelry drawer holds not only one pearl, but five, the other four are imitations. Barry managed to take the original from the pearl. Is Barry's choice just a luck? Not at all. He carefully rubbed the pearl into his teeth. Barry says fake pearls are smooth and slick, while the original pearls give a slightly rough sensation, like grate.