Luminous marking in an indentation of an object

Luminous marking on an object, said object having an indentation therein, comprising a phosphor in said indentation, a source of nuclear radiation in said indentation and an artificial, transparent sapphire crystal shaped to conform to the mouth of said indentation and to form a seal for confining said phosphor and source of nuclear radiation within said indentation and for preventing entry of corrosives or acids from the exterior, the outer surface of said sapphire crystal being essentially flush with the outer surface of said object at said indentation.