Silver futures fell to a six-week low as the dollar’s advance reduced the appeal of commodities as alternative investments. Gold, platinum and palladium dropped.

The greenback rose to a five-month high against a basket of 10 currencies amid signs of gains in the U.S. economy. In July, sales of silver coins by the U.S. Mint, the world’s largest, tumbled 27 percent from June to the lowest this year. The Bloomberg Commodity Spot Index of 22 raw materials dropped to the lowest since February.

“The dollar is pushing the precious metals lower,” Bart Melek, the head of commodity strategy at TD Securities in Toronto, said. “Also, there is little physical demand for silver.”

Silver futures for September delivery tumbled 2 percent to settle at $19.833 an ounce at 1:40 p.m. on the Comex in New York, below the 100-day moving average. The price touched $19.78, the lowest for a most-active contract since June 18.

Last year, silver and gold plunged the most in three decades on concern that the Federal Reserve would slow the pace of U.S. monetary stimulus as the economy rebounded.

On the Comex, gold futures for December delivery fell 0.3 percent to $1,285.30 an ounce. Trading was 30 percent below the 100-day average for this time, data compiled by Bloomberg show.

Gold for immediate delivery rose 0.3 percent to $1,291.71 at 2:51 p.m. President Vladimir Putin ordered the government to prepare a response to U.S. and European sanctions as Russia said eastern Ukraine approached a “humanitarian catastrophe” and required immediate international assistance.

On the New York Mercantile Exchange, palladium futures for September delivery dropped 0.8 percent to $848.35 an ounce.

The price has climbed 18 percent this year after a five- month strike by miners cut output in South Africa, the world’s second-biggest producer. Russia is the top source of the metal, used mostly in pollution-control devices in cars.

Platinum futures for October delivery fell 0.7 percent to $1,455.90 an ounce. Earlier, the price touched $1,451.10, the lowest since June 23.