What is the philosopher's stone

"The philosopher's stone is a legendary substance, allegedly capable of turning inexpensive metals into gold. It was sometimes believed to be an elixir of life, useful for rejuvenation and possibly for achieving immortality. For a long time, it was the most sought-after goal in Western alchemy. In the view of spiritual alchemy, making the philosopher's stone would bring enlightenment upon the maker and conclude the Great Work. It is also known by several other names, such as 'materia prima.'"