philosopher's stone

noun2

1. •
   A supposed substance able to turn base metals, such as lead or mercury, into gold or silver, also sometimes claimed to cure any illness (as panacea) or confer immortality (as elixir of life), among other functions.

   • Men talke much […] of the Philosophers stone, that it turneth copper into gold; […] — 1611
2. •
   A piece of such a substance.