Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some global materials stocks to your portfolio but don't have the time or expertise to hand-pick a few, the iShares S&P Global Materials ETF (NYSEMKT: MXI) 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 0.48%. It recently yielded more than 2%, too.

This ETF has underperformed the world market over the past three and five years, but the future matters more than the past. And as with most investments, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

Why materials?
Our global economic slump won't last forever, and there are already signs of life here and there. Thus the materials industry is poised to prosper as construction and infrastructure projects get underway and manufacturing kicks into a higher gear.

More than a handful of global materials companies had strong performances over the past year. Air Products (NYSE: APD), which is focused on industrial gases and chemicals, surged 33%. Activist investor Bill Ackman has been buying shares, which led the company to adopt a "poison pill." (Ackman's interest might relate to the company's status as the largest producer of helium, which is in short supply lately.) Meanwhile, the CEO is departing in 2014, which adds some uncertainty to the company's future. The stock yields 2.7%, and the dividend has been hiked by an annual average of more than 10% over the past five years.

Other companies didn't do quite so well over the last year but could see their fortunes change in years to come. Fertilizer giant PotashCorp (NYSE: POT) sank 27%, making it only more attractive to those who see it serving a growing global need for food. Bulls like its low-cost structure and solid profit margins, and they even see its ultimate success as almost inevitable. Still, it's vulnerable to fertilizer prices and the state of developing economies, such as China, where growth has slowed, and India, where there has reportedly been a potash oversupply. New competition may come from BHP Billiton, which is building a potash mine in Canada. PotashCorp carries significant debt, , but it's also generating more than $1 billion in free cash flow annually. It recently yielded 4.4%, and its dividend has been hiked 25% this year and some 700% over the past few years.

Freeport McMoRan Copper & Gold (NYSE: FCX) shed 10% and yields about 3.7%. The world's largest publicly traded copper-producer has been challenged by falling copper and gold prices, and faces new challenges, too, such as an export restriction in Indonesia that will affect its production there. Freeport's second quarter was encouraging, featuring estimate-topping earnings (and revenue a bit below expectations), along with effective cost-cutting. Bulls like its recent purchases of oil and gas exploration and drilling companies, which significantly diversify its operations. While some wait for better days, others see Freeport's stock as appealing.

Brazil-based Vale (NYSE: VALE), the world's largest iron-ore concern, dropped 8%. The company posted second-quarter results featuring estimate-topping earnings but revenue below expectations. Management expects improvement, though, due to production increases and effective cost-cutting, which includes sales of non-core assets. Meanwhile, the company has been becoming a bigger player in copper. Some worry about continued pressure on commodity prices, while others see the stock as undervalued, with its forward P/E near seven -- about half of its five-year average of 14. The stock yields about 4.8%.

The big picture
Demand for global materials 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.

More on ETFs
This isn't the only interesting ETF out there. You can learn more about a few ETFs that have great promise for delivering profits to shareholders by checking out The Motley Fool's special free report "3 ETFs Set to Soar." Just click here to access it now.

Longtime Fool contributor Selena Maranjianwhom you can follow on Twitter, has no position in any stocks mentioned. The Motley Fool owns shares of Companhia Vale Ads and Freeport-McMoRan Copper & Gold. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.