I wouldn't even make the claim that silver and gold functioned as money. In pretty much all circumstances that I'm aware of, the scarcity (in the slightly unconventional sense I'm using it) and fungibility of gold and silver money is not great -- debasing with cheaper, more common metals resulted in differing standards of purity.
Most often, when gold and silver were used as currencies, it was because some dude punched his profile on one side of the coin, and because doing that as a private citizen was difficult and/or punishable by gruesome torture, the value of the money was related to how powerful the head on the coin was because that related to the probability that the coin wouldn't turn green.
This doesn't really answer the question you posed. Now I'm totally in the realm of post-hoc reasoning, though, and I can't really think of a concrete way of falsifying this argument, but I'll go for it anyway. Gold and silver occur naturally near the surface in relatively pure forms; gold especially since it doesn't really allow with anything. As a result, these metals were present in antiquity, and as systems of money grew up, they were good candidates because people thought they understood the scarcity -- an understanding that turned out to be proven incorrect several times in the history of coinage, with reasonably disastrous impacts on savers in those metals. Of course, "being used in antiquity" was not one of the properties I defined above. Arguably fungibility is a factor if the metals were not easily identifiable or distinguishable from other metals.