Philosopher's stone

The philosopher's stone is a chemical substance capable of turning base metals into gold. Its formulae was written down by Nicolas Flamel in the first pair of journals that formed the Book of Abraham, "True Magic", detailing the learnings of Abraham of Würzburg.

In 1520, True Magic was in the possession of the Swiss alchemist Bombastus. However, only his friend Giovanni Borgia, a descendant of the First Civilization, was able to comprehend the formulae written down, and made notes of it. Bombastus then made the substance written down in Giovanni's notes, rediscovering the philospher's stone.

Reference

 * Assassin's Creed: Project Legacy - Rome: Chapter 4 - Giovanni Borgia