Rubedo, the reddening, is the fourth stage and the completion of the Magnum Opus. It is the reunion of purified spirit with purified body, of soul with matter, yielding the Philosophers Stone, called also the red stone, the red king, and the elixir.
The colour-stages of the Work
Symbolically Rubedo marked the union and perfection of the whole, the attainment of the goal figured as the Philosophers' Stone or the red king. The classic sequence, nigredo, albedo, citrinitas, rubedo, describes the transformation of base matter into the perfected end of the Work through a succession of colours seen in the vessel.
The red stage is frequently pictured in alchemical emblem literature as a marriage, the chymical wedding of a red king and a white queen, or of sun and moon, imagery that runs through texts such as the sixteenth century Rosarium Philosophorum and its many illustrated successors, and which the depth psychologist C. G. Jung, in his 1944 study Psychologie und Alchemie and in the later Mysterium Coniunctionis, took as the clearest alchemical picture of what he called the union of opposites, the psychological integration he saw the whole colour sequence as figuring. The stage is described here within that documented tradition of European alchemy and its later interpretation.
Sources The Secrets of AlchemyLawrence M. Principe with Psychology and AlchemyC.G. Jung