More recently, weakness in top holding Freeport-McMoRan (NYSE: FCX) has weighed on the materials ETF. Other funds in the sector include iShares Dow Jones US Basic Materials (NYSEArca: IYM), Focus Morningstar Basic Materials (NYSEArca: FBM) and Vanguard Materials (NYSEArca: VAW).

Lackluster performance from copper ETFs could be another warning sign for U.S. stocks. Copper demand has been indicative of economic activity because of the base metal’s wide industrial applications. [Paging Dr. Copper]

Materials Select Sector SPDR Fund

iPath Dow Jones –UBS Copper ETN

Full disclosure: Tom Lydon’s clients own FCX.