The Methuselah Stone
An ancient stone believed to magnify the Quickening power of whoever possessed the completed stone, according to legend.
In Legacy, Amanda said, "Can you imagine something that increases an Immortal's power tenfold?" though she was later heard to say, "Those crystals can make an Immortal totally invulnerable!" Later, Duncan reiterates, "Whoever has the crystal becomes invincible."
Methos, in Methuselah's Gift, mused about Luther's obsessive search for the Stone; "The thought of you becoming all-powerful scares the hell out of me even now." Then asked, "But is it really possible that the possession of all the pieces could make an ordinary human immortal? Only one way to find out." He then told MacLeod that, "Whoever has it lives forever."
As with any mythological relic like the Methuselah Stone, no one was quite certain what the stone is supposed to be capable of. "Some say that the crystal will protect its wearer from death," related Watcher, Nathan Stern.
Rebecca Horne possessed it for many years and gifted her students with individual crystals from it. As she told young Amanda, "When each of my students leaves me, they... take a piece of this with them. It keeps us together. One day, I will tell you all about it." Luther, a former student, eventually hunted her other students for their crystals, and killed her in pursuit of the completed stone. After Luther's death, the incomplete Stone was recovered and kept on display in the Director's Gallery at Watcher Headquarters.
When Methos became desperate to save Alexa Bond's life, he too, turned to the legend of the Stone and tried to steal it from Watcher HQ in bid to save her. "Do you know anything about the Methuselah Stone, MacLeod? There have been stories about its power since before I was born! Who ever has it, lives forever....If there is one chance that I can save her, then I have to try." Eventually, Methos teamed with Amanda, and together they managed to steal the Stone (Methuselah's Gift).
The Stone, however, was lost in a French river when Watcher, Daniel Geiger, attempted to steal it for himself; he managed to assemble the complete Stone for a few seconds which morphed from a jagged rocky lump to a multifaceted crystal sphere prior to him being shot and killed by Amanda. Geiger's body was eventually recovered from the river, with a piece of the Stone clutched in one hand.
The Stone or its crystals was seen or referenced in Legacy, Finale: Part Two, Methuselah's Gift