Historically, only a small number of retail jewelers bought diamonds from the public. In the 1990s, however, the advent of Internet price lists and online retailing began to put the squeeze on margins for brick-and-mortar merchants, so traditional jewelers, looking for ways to compete with their Web-based peers, began to give the secondary market a closer look.More recently, supply issues have made diamond buying from the public more lucrative. But the real trigger was this year’s tremendous price volatility. “Never before in my 26 years of buying diamonds have I seen diamond prices rise so sharply, so fast,” says John Sabet, owner of Charleston Alexander Diamond Importers, with two stores in Bethesda, Md., and Falls Church, Va. Over the past year or so, retailers have gone from buying melee-size goods at deeply discounted take-in prices from the public to buying single stones