The trick to lighting crystals and gems is to direct the illumination exactly where it needs to go. Flooding a cabinet or set of shelves with bright illumination will only leave the specimens appearing dull. Aiming a high power fixture at a small part of the specimen will create a tiny, but noticeable reflection. This is what gives a specimen its sparkling, brilliant appearance, so the goal is to set up a number of small fixtures around every exhibit. That may sound like a lot of work, but it makes no sense to spend thousands on several beautiful specimens only to leave them under poor illumination.
Lighting crystals and gems is best done with halogen fixtures, as they can be easily focused. Designers will take into consideration the way the specimens are being displayed before setting up fixtures, and will be able to work with cabinetry, shelves, and pedestals, no matter their layout. A set of small recessed fixtures can provide needed illumination in a cabinet, as the underside of each shelf will provide enough space to accommodate the fixtures. With shelving and pedestals, the best option is usually an optical framing projector. Projectors can be masked to produce any pattern of illumination, and can even work with several specimens at once.
A fixture designer will be able to come up with the right system for a display, and the end result will be a sophisticated, crafted look that is usually only seen in museums.