Opus alchymicum, or opus, sometimes called the «work,» is the alchemical process of transmuting cheaper metals into more precious ones.
Initially the opus alchymicum had four stages: the nigredo(black), the albedo(white/silver), the citrinitas (yellow), and the rubedo (red/gold).
After the fifteenth century references to the citrinitas stage were gradually phased out leaving the opus alchymicum with three main stages. A.G.H
Sources:
Drury, Nevil. «Opus, Stages of the» The Watkins Dictionary of Magic. London. Watkins Publishing. 2005. pp. 218-219