(Bloomberg) -- Silver jumped to an almost eight-year high, extending a surge that started with the retail-investor frenzy sweeping through markets.Most-active futures rose as much as 13% to $30.35 an ounce on the Comex, the highest since February 2013. That followed a weekend buying binge that overwhelmed online sellers of silver coins and bars from the U.S. to Australia. BlackRock Inc.’s iShares Silver Trust, the largest exchange-traded product tracking the metal, recorded an unprecedented $944 million net inflow on Friday. Late Monday, CME Group raised futures margins by 18%, making i...