Many thanks, O Prince among Dragonlords! Weeding through the various stashes of alchemy material we have here is a real labor of love.
These links are likely to stay live for quite a while, so this will be extremely helpful. But we can also post cut-and-paste chunks of writing (articles, text selections) that we find, to keep the bigger context of a quote available for our readers, even though we may only use a sentence or paragraph of a piece in a post we might make in a literature thread.
Here is the link and a small section of one of the best overviews for the modern reader that I have come across. (It also has a trippy, Blake-inspired graphic. ^_^ ) This article (originally written as the introductory reading for a college course) is attempting to tie the medieval vision of alchemical processes to the Jungian interpretation of alchemy as a process of inner psychological development (which you can see as metaphor, or, as Jung saw it, the older experiments were a projection outward into the physical realm of those same inner psychological processes which alchemists were already aware of, on some level.) Since we are already thinking of metaphoric psychological applications for alchemy within the Harry Potter books, this becomes a very useful orientation to the whole difficult business. A long-ish read, but very readable, and hard to put down if you're at all interested in these things.
http://zero-point.tripod.com/alchemy/alchemyclass.html"Grossinger says that "what Carl Jung recognized was that the stages of the alchemists also corresponded to a process of psychological individuation. The psychic stages were as precise and rigorous as the chemical ones by which they became imagined. Furthermore, they generated a physical and even quantitative terminology for an undiagnosed tension of opposites in the human psyche arising from male and female archetypes, a struggle they sought to resolve by the creative unity of the chemicals in the Stone." Alchemy sought to unite Spirit (male), and Matter (female) through a Royal Union (coniunctio) to create their synthesis in the homunculus, hermaphrodite, or lapis. This is an alchemical metaphor or version of the generic process of spiritual rebirth.
The entire body of alchemical literature covers many variations on the theme of the Great Work. No single person will ever express all of the operations and symbols described in alchemy, just as no single person ever embodies the totality of the Self. We each have unique experiences of the common roots of humanity or the collective unconscious. Thus, the various operations of alchemy come in different order for the various practitioners. The alchemical writings seem to contradict one another about the evolution of the process. Some claim to have made the Stone and lost it, over and over -- like the elusive revelations of a psychedelic trip. Likewise, in dreams we sometimes find the symbols of the end-product (like a mandala, or flower, or child) appearing at the beginning of the process. They symbolize what is latent and seeks manifestation.
Nevertheless, in both alchemy and Jungian psychology there are classic stages in the process of individuation or personal experience of the unconscious -- psychic milestones. One major recurrent theme in modern dreams is the symbolism of the planets, which correspond with the alchemical metals. These metals, or planets (astrology), archetypes (depth psychology) or Spheres (QBL) can be understood psychologically as the building blocks of the ego, which forms itself from fragments of these divine archetypal qualities. These spiritual principles seek concretization through the unique experience of an individual ego. This links spirit and matter; it comes down to earth.
The sacredness of the Opus, or Great Work, is the central idea behind alchemy. It is a holistic perspective. One must be self-oriented, rather than ego-oriented. The adept is also diligent, patient and virtuous. In other words, in order to create the Stone, you must have that integrative potential within yourself for self-realization -- for becoming whole or 'holy.' It requires an inward seeking, just like the process of individuation. It is a solitary task for no one may follow where you go. But there may be guides who will help inspire your faith and dedication to the task. Others have been to the territory you will explore, but none will accompany you.
The secret of alchemy is that it is a personal journey of transformation, and cannot be explained but only experienced. It is "eating the dish," not just reading about it in an alchemical cookbook. Its effects must be channeled into spiritual growth, for if alchemy is used to gratify personal desire the work is lost. This means the ego gets inflated with its own importance when the real power source lies within the Self. This naturally produces a regression back into an unconscious state, back to the prima materia, raw psychic material. The instinctual urge for growth and transformation lies within us. For this urge to be considered evolutional requires that the ego must cooperate quite deliberately and consciously with the Self. This leads toward self-realization.
The main purpose of the Opus is "to create a transcendent, miraculous substance which is variously symbolized as the Philosopher's Stone, the Elixir of Life, or the universal medicine (panacea). The procedure is, first, to find the suitable material, the so called prima materia (lead), and then to subject it to a series of operations which will turn it into the Philosopher's Stone (gold)." (Edinger, 1978)." 
