the stage occurring immediately after the deathly black stage or nigredo, and just prior to the pure white stage or albedo. After the nigredo, the blackened body of the Stone is washed and purified by the mercurial water during the process of ablution.
Abraham provides the canonical definition of the peacock’s tail as the iridescent, rainbow-coloured phase interposed between nigredo and albedo, occasioned by ablution of the blackened prima materia.
, A Dictionary of Alchemical Imagery, 1998thesis