Crystals are a key element in the design and function of the Christa.
Initially, all of the controls seen on the Christa are crystals: as the controls in the Command Post, at the Power Junction, in the Engine Room, and elsewhere. Eventually the crew (or the ship herself) changes some of these objects to make them more manageable for the people using them.
Thelma is powered on by a small diamond-cut crystal that fits into a notch on her upper forehead. It also serves as a memory chip. Harlan finds Thelma's memory crystal when he steps on it; the resulting cracks damage her memory.