A quick-response, or QR, code is a mystery to many. But “the little box with the squiggly lines,” as user Courtenay Madsen describes it, is how jewelers on the cutting edge are connecting with customers. “Most people just know  that they can scan it,” says Matthew Perosi, CEO of the Jeweler Website Advisory Group in Totowa, N.J. But its potential, he says, is so much greater.What are they, anyway?QR codes were developed by Japanese automakers as an alternative to ordinary bar codes. They were used in manufacturing before ­attracting the attention of the advertising industry, which realized that the large amount of information QR codes could hold had significant potential in the growing field of mobile marketing.To get them to work, you download a QR reader app onto your smartphone, then snap a picture of the code with the phone’s camera. The directions embedded in those s