Tungsten, mainly. Anti-counterfeit testing now includes ringing the purportedly gold coin like a bell, if it passes the density test. A clad-tungsten counterfeit will have a duller, less sonorous sound.

This is where all those hours of listening to drummers drone on and on about the metallurgical qualities of cymbal brass--and the audible differences between Paiste, Sabian, and Zildjian--can come in handy.

Pyrite (aka "fool's gold"), if we're talking about the average person rather than someone who knows well how to test it.

You can turn it into a sheet and wrap it around lesser metals.