Originally Posted by patcleaver

It can be determined whether a stone with an inscription is a recent forgery by studying the exterior layer of the stone called the fascia patina, especially within the inscription. Most inscribed stones are limestone or marble and an oxide layer forms over a period of hundreds of years over the surface of the stone and inside the inscription. At least some recent forgers, have been unable to convincingly duplicate the fascia patina. The fascia patina can be compared to the fascia patina of inscriptions of known age in the same environment to get a ballpark guess of the age - may be within 20% of the age.