S4·E16 Methuselah's Gift

When masked thugs try to take Amanda's head in her sleep and steal the crystal given to her long ago by Rebecca, Amanda and MacLeod investigate. They discover the the crystal may be part of the Methuselah Stone, a mystic talisman said to impart eternal life and invulnerability to the wearer and that the person willing to kill Amanda to get it may be -- Methos?