The raw-materials rally that has driven investors to load up on gold, crude and wheat is also sparking interest in funds tied to relatively obscure commodities such as lithium, uranium and rare earths.  Investors have poured hundreds of millions of dollars into a handful of exchange-traded funds linked to those materials over the past year or so. But betting on these kinds of industrial building blocks presents some unusual challenges and risks.  Trying to replicate the price swings in underlying materials through an ETF is challenging because there are typically no futures markets for these substances, as there are for more commonplace materials. Holding the physical goods is often impractical as well. As a result, many funds instead concentrate rare-earth and other exotic-metals plays on related stocks, which can rise or fall independently of the commodities.  The fortunes of some of these materials—and the companies that work with them—can change suddenly. After Japan's nuclear disaster in March, two ETFs that hold uranium-related stocks plunged amid a clouded outlook for nuclear energy and haven't recovered to date. In addition, uncertainty about the global economy has caused prices of some rare earths to fall by double-digits in percentage terms in recent months, according to market participants.  Investors who accept the risks are generally buying into a thesis that's been applied to a broad range of commodities in recent years—that rapid economic growth in emerging markets is pushing up demand and suppliers are struggling to keep up. Indeed, some basic commodities have leaped in price, but some of the biggest increases are related to lesser-known materials.  While oil costs a little more than twice what it did at the low point in 2009, for instance, the price of neodymium—one of a group of rare-earth elements used in high-tech products and advanced weaponry—was recently up 23-fold over a similar period, according to American Elements, a Los Angeles manufacturer that uses rare earths.  Van Eck Global last year launched Market Vectors Rare Earth/Strategic Metals. What qualifies as a "strategic" metal is "a little subjective," says marketing director Edward Lopez. But instead of buying the metals, the fund buys shares in companies that get at least half their revenues—or have that potential—from rare earths or materials such as titanium and tungsten.  Bolivia has a pilot plant to extract lithium from beneath vast salt flats  Despite their name, rare earths are common in the Earth's crust. But about 90% of rare-earth supplies currently comes from China, which has started to limit exports, saying it needs the materials at home. Likewise, foreign investors face restrictions on holding shares of major Chinese rare-earth producers, Mr. Lopez says.  Mining companies in the U.S. and elsewhere are trying to ramp up production to replace lost supplies. Investing in such companies carries distinct risks, Mr. Lopez says, including the hurdles of moving from planning to production and the possibility that the market for the materials may shift in the meantime. But the Van Eck fund includes among its top holdings Molycorp Inc., in Greenwood Village, Colo., and Australia-based Lynas Corp., companies that are developing rare-earth mines in the U.S. and Australia, respectively.  Shares of the Van Eck fund are down 21% since it was launched last October, and down 36% this year through Sept. 30. The fund at the end of August had $346 million in assets, according to National Stock Exchange, a data provider and stock exchange.  Lithium is another metal that has attracted widespread interest, because of the vital role it plays in powering a proliferating array of consumer electronics, including cellphones and laptops. But, as with other such elements, it's impractical to invest in lithium directly. It's an often volatile material and insuring a large stock could "take so much away from the return that it wouldn't be practical," says Bruno del Ama, chief executive of Global X, an ETF provider.  The company's Global X Lithium, launched in July 2010, invests in shares of companies that mine lithium and in makers of products that use lithium, such as lithium-ion batteries.  The fund's largest single holding is Sociedad Quimica & Minera de Chile SA, a Chilean company that produces plant nutrients and iodine as well as lithium. Shares in the company made up 23% of the fund's holdings as of Sept. 30.  The fund had $128 million in assets at the end of August, including inflows this year of $24 million, according to National Stock Exchange.  Mr. del Ama says buying stocks can give investors a boost because miners can make money even if prices for the material stay flat. "If on top of that, the price of the commodity goes up…you get a leveraged impact on the return," he says.  Shares in the lithium fund have fallen 16.2% since the 2010 launch, and are down 41% this year through Sept. 30. Average lithium prices in 2011 through July were 2% below average prices last year, according to TRU Group Inc., a consultancy that specializes in lithium.  The recent fate of two uranium-linked funds—Global X Uranium and Market Vectors Uranium+Nuclear Energy—shows that the "leveraged play works both ways," as Mr. del Ama puts it.  After the March 11 earthquake and tsunami in Japan crippled the Fukushima Daiichi nuclear plant, uranium prices plunged amid concern the incident would undercut support for nuclear power. In early September, weekly prices for the thinly traded fuel were 23% lower than they were on March 7, before the disaster, according to Ux Consulting Co. LLC.  But shares in Global X's uranium fund, which focuses on uranium mining, have fallen even harder, losing more than half their value since March 10, the day before the Japanese disaster. The Market Vectors fund, which invests in both miners and other firms that work on nuclear energy, has fared somewhat better over that same period, falling 33% through Sept. 30.  Mr. Pleven is a reporter for The Wall Street Journal in New York. Email him at liam.pleven@wsj.com.
Exchange-traded funds are jumping on the bandwagon to invest in rare earths and other strategic metals, mainly by investing in companies that mine and use the materials. There are risks for ETF investors to weigh.