Brewer's: Elixir of Life
A ruby, supposed by the alchemists to prolong life indefinitely. The tincture for transmuting metals was also called an elixir. (Arabic, el or al iksir, the iksir (? coction).) (See Amrita.)
He that has once the Flower of the Sun, The perfect ruby which we call Elixir ... Can confer honour, love, respect, long life, Give safety, valour, yea, and victory, To whom he will. In eight-and-twenty days I'll make an old man of fourscore a child.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894