The Philosophers' Stone
(Latin: lapis philosophorum; also called variously an elixir or tincture)
In alchemy, a magical substance, whereby base metals can be transformed into noble metals (notably, gold). In alchemical thinking, a base metal is a condition of "disease" in a noble metal, a condition which can potentially be cured. The philosophers' stone is thus a sort of universal curative, which can heal both sick metals and sick people (by implication, ensuring eternal life, or at least great longevity).
The stone (of which it was generally assumed that there could only be one) was considered the alchemical center of the world, axis mundi, and would be composed of a perfectly balanced distribution of the four alchemical elements: earth, air, fire and water
For over 2000 years, different alchemists have claimed to be in possession of the philosophers' stone, or the means for creating it. With the decline of alchemy as a serious field of study, the stone has achieved a different significance. In analytical psychology, C.G. Jung interpreted the philosophers' stone as a symbol of the self.
Note: Take a moment's time to parse the Latin version, listed above, paying close attention to the fact that philosophorum, "of the philosophers", is plural, a fact that somehow seems lost on most writing on this subject. For some reason, few people grasp that this object is necessarily both unique and the subject of the interest of many of the Wise, not just one.