Page "Gold coin" Paragraph 86

Unlike silver, gold is denser than almost all other metals, hence whether something is made of gold is extremely hard to fake.

Simple determination of weight and volume should be sufficient.

A coin that is the right size but is not gold, or has too much base metal, will be " light "; alternately, a coin that weighs right will be somewhat larger.

Most metals that are of similar or higher density than gold are similarly or more expensive, and were unknown in ancient times ( notably the platinum group ); only two relatively inexpensive substances are of similar density to gold: depleted uranium and tungsten.

Of these, depleted uranium is government-regulated, but tungsten is more commonly available, hence more suited for counterfeiting.

Alloying gold with tungsten would not work for several reasons but a coin with a tungsten center and gold all around it could not be detected as counterfeit by density measurement alone.

This method ( tungsten wrapped in gold ) has found some use in counterfeiting of gold bars.