What is the philosopher's stone?

A person looking for a short cut to riches is said to be searching for
the philosopher's stone. Ancient alchemists believed somewhere in
nature there existed a substance which would convert ordinary metals
into gold.

This imaginary substance was called the philosopher's stone because it
was supposed to have a philosophic basis and was linked with the
theories of matter advanced by philosophers.

The idea of transmuting base metals into noble ones appears to have
originated among the Greeks at Alexandria in the early centuries of the
Christian era. In medieval times, the philosopher's stone was reputed
not only to have the property of transmuting the baser metals into gold
but also the power of curing most of the ills that the body is prone to.