philosopher's stone

Short definition from Wiktionary

Noun

1. 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.
2. 2
   A piece of such a substance.
3. 3
   Alternative letter-case form of philosopher's stone.