Alchemy.
A subject that Leviticus was very fascinated with. It is a branch of magic that deals with equivalent exchange; it is also the ancient forerunner of mediums, based on the supposed transformation of mana to gems. It is concerned particularly with attempts to convert minerals or mana or to find a universal elixir, or in this case, a gem.
It is the branch of magic that was quite more intriguing than the rest, in Leviticus' case. Mediums of magics are usually created from seeds that materialize one's personality, origin, and mana. That was the textbook and general explanation about the mediums.
But alchemy gives another definition.
Alchemy is primarily concerned with how rather than why. It explains how mediums are created from mana and how it forms particularly. Alchemy also explains how incantations are created and how hexes correlate with words. Rather than giving a one-liner explanation about things, alchemy explores the tiniest of the details.
