(FORTUNE Magazine) – Whenever an investment category gets hot, you can bet that the financial-services industry will roll out new products to capitalize on the trend. Case in point: the new batch of exchange-traded funds (ETFs) focused on commodities. (Unlike traditional mutual funds, ETFs are bought and sold like stocks at prices that vary throughout the trading day.) In late April, Barclays Global Investors introduced iShares Silver Trust (SLV), an ETF that tracks the price of silver, which recently topped $14 an ounce, up $7 since September. One share of SLV represents ten ounces of silver; the fund's assets consist of bullion stored in London vaults by its custodian, J.P. Morgan.

The silver fund follows on the heels of United States Oil (USO), an ETF that tracks the price of light sweet crude and began trading in April. Both hope to emulate the success of the two gold ETFs that have proved highly popular: Barclays' iShares Comex Gold Trust (IAU), launched last year, and State Street's year-and-a-half-old StreetTracks Gold (GLD).

The advantage of these ETFs is that they give individual investors a convenient and relatively inexpensive way to invest in commodities. But the fact that it's easy doesn't mean it's smart. "Typically, the time to think about adding something like commodities to your portfolio for diversification purposes is when they're out of favor," says Dan Culloton, a Morningstar analyst who specializes in ETFs. "And that's not the case right now. The history of metals is pretty volatile. If you're thinking of adding something like this, you really have to have an extremely long time horizon and an extremely strong stomach."