Exchange-Traded Funds; Some ETFs Boom by Getting Physical; Funds holding gold bullion are popular. But there are challenges to investing directly in other metals and commodities.

2006, when silver cost less than $10 a troy ounce, an executive at a U.S. unit of Fujifilm Holdings Corp. wrote to the Securities and Exchange Commission, saying a fund backed by silver, a key component of photographic film, would "mean higher product...