Copper is more than the main ingredient in wire and gold is more than what we wear on our fingers and around our necks. These commodities, along with others like oil and grains, are used by investors to gauge the health and short-term direction of the market, but how does it work
Gold
Gold is the best-known commodity because it appeals to investors and non-investors alike. Consumers may not think of gold as an investible product, but the story of gold is actually complicated. Not only does it serve as a commodity, but also as a currency. In the latter part of 2011 and into 2012, it has taken on the behavior of a stock often mirroring the overall market.
Traditionally, gold tends to move in the direction opposite the market. Investors use gold as a market hedge, dumping money into the commodity when the market is trending lower. In times when it is acting like a commodity, investors watch gold closely. When they see money pouring into GLD, the ETF that tracks the performance of gold or gold futures markets, they believe that a market downturn may close at hand.
Copper
Copper doesn't have the allure of gold since it's a base metal used largely for industrial purposes, but that doesn't change the fact that investors watch it closely for hints of the overall market sentiment. Because Copper is an industrial metal, investors use it as a way to gauge the health of the manufacturing and housing sectors of the world's economies.
Investors also use Copper as a way to gauge trader sentiment. When copper is rising, some see that as investors having an appetite for risky assets, since Copper is known as a volatile commodity. When copper loses value, it may indicate that investors are selling risky assets and a market correction may be imminent.
http://www.investopedia.com/financial-edge/0312/how-commodities-predict-market-movement.aspx
