It's a metal whose performance tends to track the S&P 500 — copper. And over the past two weeks it's been falling like lead.
So with stocks soaring, what's the problem?
(Read More: Stocks Lower on China; AAPL at 52-Week Low)
Well, copper futures have been driven down by attempts by the Chinese government to take the air out of an impending real estate bubble. Stricter down payment ratios and higher taxes on speculative property are among several measures that could be implemented.