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
Drury, Nevil. “Opus, Stages of the” The Watkins Dictionary of Magic. London. Watkins Publishing. 2005. pp. 218-219