What is the difference between a Silver ETF and Sensex/Nifty index?
Sensex/Nifty are indices comprising 30 and 50 stocks respectively in a certain proportion. The index funds that replicate them invest in stocks in the same proportion as the different companies' weight in these indices. So, these funds get you a return exactly that of the index, subject to expenses and tracking inefficiencies.
On the other hand, in the case of silver ETFs, all your money is invested in silver. Your return is the appreciation in the price of silver after accounting for the expense ratio of the exchange-traded fund (ETF) and the inefficiency between ETF price and silver price. So basically, here, you invest in silver compared to investing in a bunch of companies.