A notice explains what I take from the Wikipedia: In Greek mythology, Charon or Kharon (/ˈkɛərɒn/ or /ˈkɛərən/; Greek Χάρων) is the ferryman of Hades who carries souls of the newly deceased across the rivers Styx and Acheron that divided the world of the living from the world of the dead. A coin to pay Charon for passage, usually an obolus or danake, was sometimes placed in or on the mouth of a dead person. Some authors say that those who could not pay the fee, or those whose bodies were left unburied, had to wander the shores for one hundred years.

I came across some discussion regarding it always being money, but gold leaf could be used too. Again in Wikipedia: Although archaeology shows that the myth reflects an actual custom, the placement of coins with the dead was neither pervasive nor confined to a single coin in the deceased's mouth. In many burials, inscribed metal-leaf tablets or exonumia take the place of the coin, or gold-foil crosses in the early Christian era.