Dilithium

In the Star Trek universe, dilithium is a fictional chemical element, although the name also applies to a real-world molecule. Dilithium typically occurs as an extremely hard crystal mineral, which occurs naturally on some planets. When placed in a high frequency electromagnetic field, magnetic eddies are induced in its structure which keep charged particles away from the crystal lattice. This prevents it from reacting with antimatter when so energized, because the antimatter atoms never actually touch it. Therefore, it is used to contain and regulate the annihilation reaction of matter and antimatter in a starship's warp core, which otherwise would explode from the uncontrolled annihilation reaction. Though low-quality artificial crystals can be grown or replicated, they are limited in the power of the reaction they can regulate without fragmenting, and are therefore largely unsuitable for warp drive applications. Due to the need for natural dilithium crystals for interstellar... full article at wikipedia