Precious metals mutual funds have given up most of their edge over the S&P 500 since their performance peak at the end of 2010. Since Sept. 30, 1999, a $10,000 investment in the average precious metals fund would have grown to $26,076 vs. $20,374 for the widely watched stock market index, according to Morningstar data. But a sharply strengthening dollar since June 30 this year has put downward pressure on gold and silver mining stocks.