Illestine

Illestine is a crystal that forms under extreme conditions, even by most standards for crystals. Born mostly in depths of volcanoes, its difficult to work with and even more difficult to retrieve. It grows submerged within lava in red star-shaped patterns, and has an incredible resistance to the elements.
Utilized almost exclusively in its raw form, the conditions in which it can be molded are often far too expensive to be reasonable for most artisans and mages. As a result, any refinement to illestine skyrockets its value, and even then what it refined illestine can do is thoroughly untested. Luckily, its raw form contains enough strength and usability to make such processes largely irrelevant to most purposes.
Its core structure acts as a pseudo-transistor, allowing it to mold elemental energies, amplify them or even absorb them. This allows it to be used in resonance with evocation style magic, either boosting their power dramatically or even allowing you to potentially alter the energies itself.