In early 2008 it was reported that at least some of the gold bars in the vaults at the National Bank of Ethiopia were fake. The discovery was made when bars shipped from Ethiopia to South Africa were returned after they were identified as being gilded steel.
Gilded steel is a very unconvincing form of fake gold because the density of the iron alloy is significantly less. . . . There are two metals that are suitable, from both a density and economic perspective, for manufacturing fake gold - uranium and tungsten.
A Chinese company called Chinatungsten is advertising imitation gold merchandise on its website. The following quote is taken directly from their Tungsten Alloy for Gold Substitution page:
"a coin with a tungsten center and gold all around it could not be detected as counterfeit by density measurement alone ... We are well accustomed to exploit more innovative applications of tungsten products. Gold-plated tungsten is one of our main products."