Alchemists claimed the ability to change base metals into gold by means of the philosopher's stone, a substance prepared in their laboratories. The myriad scientific, pseudo-scientific, metaphysical, and philosophical theories inspired by the notion have filled entire books - here offered is a simple overview of the concept (at least those aspects not already covered under alchemy), as it pertained to practical Medieval alchemy.
In late Antiquity, techniques of coloring metals by the application of volatile substances (i.e., turning copper a silvery color by the application of arsenic) were well known. These techniques led to speculation that density, ductility, etc., could also be altered, perhaps in a similar fashion - and that real gold (considered "the perfect metal") could thereby be generated.
The creation of the philosopher's stone was envisioned as a generative rather than fabricant process, in which the stone would be incubated and finally "born" out of a kind of union between Art and Nature - a union which was seen as sexual, or was at least commonly represented through sexual allegory. Ancient Egyptian alchemists described their procedures in ellipses of agrarian metaprose, while Medieval alchemists went straight for the good stuff:
"There was a red Lion, a bold youngster, married to the Lily in the lukewarm bath; And then with an open fire, chased from one bridal chamber into another. Next, with motley colours, the young Queen appeared in the glass vessel."
The "motley colors" in question were black, white, motley, and red (in sequence), more poetically denoted as the raven, the swan, the peacock, and the lion.
See also: alchemy
, for an overview of general alchemical process and method.
Source: Reijer Hooykas, Fact, Faith, and Fiction in the Development of Science