Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some materials stocks to your portfolio but don't have the time or expertise to hand-pick a few, the Vanguard Materials ETF (NYSEMKT:VAW) could save you a lot of trouble. Instead of trying to figure out which materials stocks 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. This ETF, focused on materials stocks, sports a very low expense ratio -- an annual fee -- of 0.14%. It recently yielded 1.8%.

This materials stocks ETF has outperformed the world market over the past five and 10 years. 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 materials stocks?
Our global economy is slowly turning itself around. Thus materials stocks are poised to see their fortunes improve as construction and infrastructure projects get underway and manufacturing kicks into a higher gear.

More than a handful of materials stocks had strong performances over the past year. Alcoa (NYSE:AA), for example, surged 55% and yields 1%. It has been struggling in recent years, but its just-reported first-quarter results featured adjusted earnings nearly twice as strong as expected, with its CEO stating that "transformation is accelerating." Alcoa is focusing more on its "value-added" segments, which are focused on the rebounding auto industry, aerospace, and beverage cans. One major challenge for the aluminum industry has been a slowdown in China.

Praxair (NYSE:LIN) jumped 18% over the last 12 months and yields 2%. The company specializes in gases and surface coatings. In January, the company reported its fourth-quarter results, featuring earnings up 15%, an 8% dividend hike, and up to $1.5 billion to be spent on stock buybacks. Revenue rose by 7.5%, and operating cash flow hit a record level of $2.9 billion for the year. Bulls are hopeful about significant future growth coming from Asia.

Freeport McMoRan Copper & Gold (NYSE:FCX) gained 14% and yields 3.8%. The company offers substantial diversification beyond gold, as it's the world's second-largest copper-producer, and it also bought several big oil and gas exploration and drilling companies. Its latest oil news is that it will explore off the coast of Morocco. Bulls see much to like about Freeport McMoRan Copper & Gold, such as its relatively low production costs, stable reserve valuations for copper and gold, and growth prospects in China. Indonesia has posed problems for the company, levying taxes on copper exports so high that it stopped shipments, but it recently struck an agreement with the country.

Other materials stocks didn't do quite so well over the last year but could see their fortunes change in years to come. Newmont Mining Corporation (NYSE:NEM), a copper producer, sank 31% and yields 2.5%. It has also been affected by trouble in Indonesia, but it has a good chance of emerging victorious there. Deutsche Bank recently upgraded Newmont Mining from sell to hold. That's a definite improvement but not a ringing endorsement. The bank was pleased with Newmont's recent dividend cut. Investors have also been pleased with its debt refinancing.

The big picture
If you're interested in adding some materials stocks to your portfolio, consider doing so via an ETF. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make profiting from it that much easier.