When you fist find the crystal , there is a hint carved in the glass window, it shows the crystal, then rays over the crystal, and then the crystal emitting rays.
This hints to the crystal needing to be "charged up." So how does a light emitting crystal charge up? You hold it in the light! So you need to move the thing to an opening in ceiling where light shines through.
Take it one step at a time, once you get there, you'll see what happens next.