What is an alchemist in Coelho's The Alchemist?

Alchemy within the context of this book is a metaphor for one's transformation as a human being, a kind of transcendence from a "base" metal, the ordinary human person, to an elevated form, in alchemy, gold, but metaphorically, for self-actualization, which is the state of being one's own best self, using one's passion, talents, and experiences to their maximum.  Self-actualization is what the alchemist is trying to help the young boy achieve.  As a metaphor this works on another level as well, since there is a certain amount of serendipity involved in science (which alchemy was considered once upon a time), and there is a certain amount of serendipity involved in self-actualization, too.