Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the metals and mining industry to thrive as the global economy recovers and infrastructure and manufacturing work picks up, the SPDR S&P Metals & Mining ETF (NYSE: XME) 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 a lot of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The SPDR ETF's expense ratio -- its annual fee -- is a relatively low 0.35%.

This ETF has not performed wonderfully, lagging the world market over the past three and five years. It's the future that counts the most, though, and the past few years have been tough ones for the global economy. 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.

What's in it?
Few metals and mining companies had strong performances over the past year, as the global economic recovery is not yet in full swing.

Freeport-McMoRan Copper & Gold (NYSE: FCX), down 34%, has struggled lately, and has faced labor strife and falling copper prices. Investors might also frown at what Mike Sunnucks at Phoenix Business Journal recently pointed out -- annual compensation of $30 million or more for two top executives, and golden parachutes should the company be bought. It also doesn't help that China's economic growth might be slowing and that India's isn't firing on all cylinders, either. Of course, some think that the stock has fallen so far that it's now attractive.

Walter Energy (NYSE: WLT), down a whopping 60%, produces metallurgical coal for the steel industry, among other things. Several Wall Street analysts upgraded the stock's rating in May, and while many institutions have sold shares, an insider recently spent almost a million dollars buying shares. There can be nothing but bullishness behind that move. Walter's earnings took a hit recently, but revenues have been growing at an accelerating clip.

AK Steel (NYSE: AKS), down 59%, has had earnings in the red in recent years, increasingly so. Cash levels have also been falling, and though its 3.4% dividend yield may be attractive, it doesn't look too sustainable given the company's current condition. Amid all this gloom, though, a Morningstar analyst recently slapped the stock with a five-star rating, deeming it inexpensive.

Then there's Titanium Metals (NYSE: TIE), down 36%. Its revenue has grown about 22% over the past year, while earnings surged 41%. Titanium is used in the aerospace industry, so the fact that Boeing has finally brought its Dreamliner to market bodes well for this company. (Fully 15% of the plane is composed of titanium.) Titanium Metals has been increasing its order backlog, as well.

The big picture
Demand for metals and mining 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.

If industrial stocks interest you, check out our special free report, "The Future Is Made in America," which details a new technology that might shift the "Made in China" trend toward the U.S. instead.