Silver exchange-traded funds (ETFs) tumbled up to 4% on Friday as silver prices dropped following the Indian government issuing permits to import 6,500 metric tons of silver over the next six months.Nippon India Silver ETF, which is widely tracked by investors looking for exposure to precious metals, dropped 4% to trade at Rs 182 apiece. Tata Silver ETF, Angel One Silver ETF, Mirae Asset Silver ETF, Motilal Oswal Silver ETF, Groww Silver ETF, ICICI Prudential Silver ETF and others also dropped around 4% each.This came as silver prices dropped.