Warning: some of your $18,000 10-ounce gold bars might actually be filled with tungsten. Gold goes for about $1800/ounce; tungsten is $1/ounce.

Chemical engineer Ibrahim Fadl, who owns a business in Manhattan's Diamond District, strips away the outer layer of a 10-ounce bar of what he thought was pure gold, sold to him by a customer at his gold refinery business.

The shell peels off like foil on a chocolate bar.

"It's got to be somebody really, really professional," said Fadl. "When I analyzed them, it showed they are tungsten."

Tungsten is a metal used to make military weaponry, drilling equipment and even jewelry. Gold and tungsten have almost the exact same density, so a substitution of metals would be difficult to detect.