That is the two faced nature of silver. It is both a commodity and a minor currency.
During good times it is strictly a commodity. When gold gets expensive it becomes poor mans gold and a currency. As official money it probably can not be used again because it is too rare and if it were uses Gresham's law (http://en.wikipedia.org/wiki/Gresham%27s_law) would prevail.

Silver like gold is traded at the currency desk and not the commodity desk at the COMEX. This is probably because of historical context. The US used to have a large silver stockpile but it is now all but gone.

http://about.ag/data/csv/stockpiles.htm