Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some metals and mining stocks to your portfolio, the iShares MSCI Global Select Metals & Mining Producers ETF (NYSEMKT:PICK) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The iShares ETF's expense ratio -- its annual fee -- is a relatively low 0.39%. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF is too young to permit an assessment of its performance. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

Why metals and mining?
Our global economic slump won't last forever, and there are already signs of life here and there. As the recovery heats up, construction and infrastructure projects get under way, and manufacturing kicks into a higher gear, demand for metals and minerals will increase.

More than a handful of metals and mining companies had strong performances over the past year. Well-regarded steel company Nucor (NYSE:NUE) gained 13%, but is facing some headwinds, such as weak demand and rising natural gas prices. Its recently reported first quarter featured shrinking revenue and plunging profits. A ray of hope is strength in the auto market, which uses a lot of steel.

Southern Copper (NYSE:SCCO) advanced 12%, though it, too, posted some unpleasant results recently, with first-quarter net profits down 20%. It's planning to nearly double its production over the next few years, and has been taking on a lot more debt.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Freeport McMoRan Copper & Gold (NYSE:FCX) shed 20%. The world's largest publicly traded copper producer recently posted shrinking earnings, though its results still topped analysts' estimates. (Its recent free cash flow is less impressive, however.) The company has diversified its operations considerably by buying a pair of oil and gas producers. It doesn't help that the price of copper has been falling, and growth in China slowing.

ArcelorMittal (NYSE:MT) shrank by 28% and recently yielded 5.2%. Along with facing a weak steel market, especially in Europe, it's saddled with more than $20 billion in debt and has to deal with weak iron ore and coal prices as well. On the bright side, the company has been working to reduce its debt, and some see it as promisingly priced now, with a forward P/E near 8. Analysts at Nomura have upped its rating from hold to buy.

The big picture
Demand for metals and minerals isn't going away anytime soon. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.