GDX, which more or less tracks the HUI, the Amex Gold Bugs Index. My rule of thumb is that the HUI will move by more or less the same number of points that gold moves in dollars, and GDX will move one-tenth of that, since it's about one-tenth the HUI. In percentage terms, this means it will be twice as volatile, which is fine. As it tracks stocks instead of the metal, it won't correlate perfectly, and on some days it will be up when the metal is down, and vice versa.