A new exhange-traded fund tracks the shares of gold-mining companies. By Thomas M. Anderson, Contributing Editor July 24, 2006 Fans of exchange-traded funds have a new way to bet on gold. For some time, they've been able to invest in two ETFs -- iShares Comex Gold Trust (symbol IAU) and StreetTracks Gold Trust (GLD) -- that track the price of gold bullion. Now they can buy an ETF that invests in gold stocks, which are even more volatile than the price of gold.The Market Vectors Gold Miners ETF (GDX), launched in May, tracks the shares of 45 gold-mining companies. Six big miners -- Barrick Gold (ABX), Newmont Mining (NEM), Gold Fields (GFI), Goldcorp (GG), AngloGold Ashanti (AU) and Freeport-McMoRan Copper & Gold (FCX) -- make up about half of the index. Historically, mining share make out better in a gold rush than bullion but fare worse in lean times. The gold miners ETF is based on the Amex Gold Miners Index, published by the American Stock Exchange. For each one-percentage point movement in gold-bullion prices, the index tended to move more than 2 percentage points between April 1, 1996, and March 31, 2006. Gold peaked in May at $730 an ounce before retreating to $620 recently. But it is still up 148% from its 20-year low of $250 an ounce in August 1999. Fear boosts gold prices. Behind the bull market in gold: Worries over inflation and conflicts in Afghanistan, Lebanon and Iraq, as well as strong demand from developing countries that use the yellow metal in jewelry. Because gold is priced in dollars, it tends to perform well when the greenback weakens. It sounds like this might be a good time to consider adding a smidgen of gold to your portfolio. But if you do buy into the Gold Miners ETF, be prepared for teeth-chattering volatility. And don't forget that gold, like most commodities, is notoriously unpredictable.