The word originally comes from one referring to the juices and saps of plants, seeming to indicate I kind of herbalism by trade. Later it became the attempt to turn metals into gold. Sometimes it is nothing more than ancient chemistry.

From my novel Quicksilver, it is an elemental form of sorcery that relies heavily on the rhythms and items of nature, as well as black magic, to effect powers in the casters, control unwitting victims, or create hybrids of creatures that would otherwise be incompatible. Most alchemists in this case are vampires.

Does anyone else use some form of alchemy in their stories? What are your systems like? What do you think of mine?

I have a mental character (not placed into a story I have told yet) who is an Alchemist. He comes from a somewhat lighter fantasy that does not demand much explanation for his magics. The character fits the usual bill of being quite eccentric, but remains in complete control of his own faculties. He brews potions for just about any desired effect, whether within a biological being (he has most practice on humans, having tested on himself extensively) or a non-biological one (a natural lock-picking chemical might be within his repertoire, or otherwise a grenade like effect of exploding chemicals would certainly be within that). He eventually accumulated so much knowledge that keeping it in books was ineffective. He eventually learned to distill facts and memories into potions, so he actually keeps a library of vials full of his own memories and records of his experiments.
So yeah, his alchemy is a bit different.

I would question that any process honesty called "Alchemy" could include black magic, but I could be wrong. I have never studied the actual definitions and history of the term. To me it is much more limited to chemical-based magics, basically sci-fi with a fantasy flair.

Well, I used some of the basis of alchemy to develop the magic, I suppose. The idea of the four elements, the seven metals, astrology, and other things that I found in alchemy illustrations. The magic is the binding element, so to speak, that activates the relations to produce mystical effects.

From my reading of history, alchemy was the semi-practical study of the nature and essence of the world as the medievals understood it. It was a broader field than chemistry and included much that we would think of as "magic" because they saw those ideas as fundamentally and indivisibly connected. (And I say "semi-practical" because unlike medieval philosophy, which it drew from but did not always agree with even to the extent that alchemists agreed with each other, alchemy involved physical experiments rather than just thought-experiments. But unlike modern science (as far as its practitioners would have us believe, anyway), alchemy did begin with certain axioms about the nature and structure of the world, such as everything being composed of the four Aristotelian elements, and any result that contradicted those assumptions was seen as clearly false because of that contradiction.) "Turning lead into gold" was simply the famous unsolved-but-supposedly-once-solved-in-the-past problem that every alchemist dreamed of solving, kind of like Fermat's Last Theorem for mathematicians (or has that been solved in the last decade or so?) or cold fusion for physicists.

At this point I don't expect to use alchemy in any of my stories, unless I make a universe where the principles of alchemy are actually accurate. (In the game-world of my strategy game, on the other hand, I do intend to make alchemy one of the possible valid development paths that players can follow.)

That's where I don't think it's a far cry to have alchemy take a turn toward sorcery, if applying the astrology of the classical planets and such works in a ritualistic connection to the results alchemy hoped to achieve. It depends on the culture in which it started, I suppose.

This sort of thing is why there's no hard dividing line between Fantasy and Science Fiction. An alchemist would think he knows (if I understand correctly) that everything is made up of (what we would call) particles of fire, water, earth, and air, in the same way that we think we know that everything is made up of "atoms" that gain most of their identity (iron is iron, silver is silver, etc.) from the number of "protons" from which they are composed, and some lesser parts of their behavior (isotopes) from the number of "neutrons." All these "mystical" things about the planets and such are simply reasoning from a few different lesser assumptions, but also one centrally important identical assumption: that the same laws apply at different scales of the cosmos. (If we showed and explained the Periodic Table to a medieval scholar, he would probably see it as more mystical than anything that related the classical elements or the seven planets to substances on Earth.)

On the other hand, pretty much anything dealing with rituals would tend to shade into sorcery or something like that, and the alchemist might or might not see a dividing line there. The "as above, so below" principle (which we still apply today on a lesser scale when we assume that the planets are governed by the same laws of motion as the balls in a game of billiards) was seen as extending into heavenly politics, about which there was in some circles much speculation ... and that, I think, even the most rationally-minded alchemist would have called "mysticism."

Good points. Another variation could be that the planets are viewed as parts of grand atoms. That could be an interesting modernization of misunderstood science. ...Misunderstood from how we think things work, anyway.

Loving all these ideas and stories.

I have one old WIP with a form of alchemy in it, and it was a briefly breezed-thru story so there wasn't as much worldbuilding as there should have been. But the story's version had to do with using natural items (plants, minerals, etc.) which were rare, unusual, had strange properties, etc. to make or change things. Potions for healing, even somewhat magical elements. The idea was that "magic" (or alchemy) was simply a science that no one fully understood yet. The Alchemist characters used it as best as they could with their knowledge, and some believed or pronounced it to be magic, but most knew it was still science, still natural, with no cosmic, paranormal qualities. *shrug* It was often frowned upon by doctors and related tradesmen as some backwoods, dark thing, but it wasn't. Maybe a kind of boring spin on things, but yeah, that's the way I wrote it.