A benchmark index in the precious metals industry that is used by investors to track the performance of companies in the industry. It was created and is traded on the Philadelphia Stock Exchange. It also is referred to as XAU, which is its trading symbol. The Gold & Silver Sector index is a capitalization-weighted index composed of the common stocks of nine companies involved in the gold and silver mining industry. The XAU was set to an initial value of 100 in January 1979. Options began trading on December 19, 1983.  A capitalization-weighted index is a stock index in which each stock affects the index in proportion to its market value.