Depleted uranium
Gold is denser than almost all other metals, hence hard to fake. A determination of weight and volume is in many cases sufficient to spot forgeries. A coin that is not gold or below the expected fineness will either have the right size but will have a lower than expected weight or it weighs right and 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). During the 19th century platinum was cheaper than gold and was used for counterfeiting gold coins. These coins could be detected by acoustic properties. Only two relatively inexpensive substances are of similar density to gold: depleted uranium and tungsten. Depleted uranium is government-regulated, but tungsten is more commonly available and suited for counterfeiting. Alloying gold with tungsten would not work for several reasons, but tungsten plated with a thin layer of gold is a common type of forgery.