The operation is described by what the matter in the vessel looks like. First the blackening, nigredo, in which the starting material is broken down and putrefied and the mass goes dark. Then the whitening, albedo, often described as a washing or a purification and associated with the moon and with silver. Then in many accounts a yellowing, citrinitas, which some authors omit. Finally the reddening, rubedo, associated with the sun and with the stone itself, which is described as capable of perfecting base metals into gold and, in some traditions, of extending life.
Around this run the standard elements: a prima materia which the texts refuse to name plainly, a sealed vessel, a fire that has to be maintained at exactly the right degree for a long period, and a sequence of operations named for what they do to the matter.
Modern history of science has recovered a good deal that earlier readers assumed was allegory throughout. Lawrence Principe and others have reconstructed alchemical recipes in the laboratory from the original texts and obtained the described results, showing that the deliberately obscure language often encodes real and repeatable chemistry rather than only spiritual instruction. That does not settle what any given author intended. It does mean that reading the whole literature as symbolic is a modern habit rather than a discovery.