Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some junior gold-miners to your portfolio but don't have the time or expertise to hand-pick a few, the Market Vectors Junior Gold Miners ETF (NYSEMKT:GDXJ) 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 Market Vectors Junior Gold Miners ETF's expense ratio -- its annual fee -- is a not-too-steep 0.55%. It also offers a hefty yield, recently above 7%.

This ETF has underperformed the world market over the past three years and so far this year. It's still relatively young, though, and the future matters more than the past. 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 junior gold miners?
Gold is not everyone's cup of tea, but some investors like to include some in their portfolios for diversification's sake. Some also favor precious metals, as some of them have more utility. With gold's plunge this year, some have steered clear, while others saw bargains. Junior gold-miners, being on the small side, can offer greater growth potential but also some greater risk. Many also mine other metals, such as silver and copper, offering diversification.

Few junior gold-miners delivered strong performances over the past year, but their depressed prices can represent more attractive entry points for interested investors. (Note that many are in dangerous penny-stock territory, trading below $5 per share.)

Endeavor Silver (NYSE:EXK) sank 58%. It's focused on silver mines in Mexico and also mines gold. Last month, due to one of its three mines outperforming, management raised silver-production projections by 20%, representing a 34% increase over 2012 levels. Gold-production forecasts were hiked 48%. Not all mines are performing well, though, and the price of silver has fallen, so the company has been cutting costs, in part via layoffs.

McEwen Mining (NYSE:MUX) slid 52%, and has also been cutting costs at Mexico-based mines. Management has suggested that the price of gold may be near a bottom, and it's looking into strategic partnerships, too. In its second quarter, McEwen upped its gold production by 30% over year-ago levels. Still, it's not yet turning a profit and is free-cash-flow negative.

Silvercorp Metals (NYSE:SVM) shed 50%, but that leaves it yielding 3.1% -- and it's even earning more than it's paying out, which is promising. The company, China's biggest primary silver producer, has been in the news as an alleged scammer as well as a possible scamming victim. (It's worth noting that it has been up front about problems, rather than evading them.) In its latest quarter, net income fell 25%, due in large part to falling silver prices, but its silver production was up 17% and gold up 42%. (It produces far less gold than silver, and it also mines lead and zinc.)

Seabridge Gold (NYSE:SA) also lost 50%. Some of its operations in Canada have been yielding high-grade materials, and its KSM Gold project in British Columbia "contains one of the largest undeveloped gold and copper reserves in the world." The company's strategy explicitly includes trying to avoid share dilution. Like many of its peers, though, it's still operating in the red, so tread carefully.

The big picture
Right or wrong, demand for gold isn't going away anytime soon, and that's a plus for junior gold-miners. A well-chosen ETF can grant you instant diversification across any industry or group of companies and make investing in it -- and profiting from it -- that much easier

Longtime Fool contributor Selena Maranjianwhom you can follow on Twitter, has no position in any stocks mentioned, and neither does The Motley Fool. 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.