Crystallomancy (Crystal gazing, Scrying) is a beautiful practice that uses a crystal to sharpen a persons mental and psychic energies to see things in the past, present or future.
Scrying comes from the Old English word descry meaning "to make out dimly" or "to reveal." Adding the prefix/suffix 'be' (often 'gye' in Germanic languages), gives us the modern word 'describe'.
Although Scrying takes on many forms, from looking into a pool of water, or a mirror to sitting on a lake side, watching patterns emerge in the waves. Crystallomancy is a more refined art. The energies of the crystal resonate with the energies of the reader (usually a clairvoyant or other type of psychically gifted person) there are many ways to charge, cleanse and keep a crystal ball. Most people will recommend buying it in person, and steeping it in moon light overnight or during a period of a full moon cycle. And storing it away, covered with a black cloth when not in use. Though these methods are not always necessary. Many people steep them in sunlight, or both, and keep them displayed out on their tables or altars to disperse the crystal's energy throughout the room.
The methods of crystal gazing also differ, some may just stare for hours on end in low lighting, while others open their third eyes and actively seek out their quarry. The main baseline is that you get into a lower brainwave state, becoming more susceptible to psychic vision and as you gaze you may see the pictures show up on the surface of the crystal, or in the holograms created by inclusions. But no matter the process. The possibilities are always limitless.
I alone have used crystallomancy to view things in the present, transfer energy, create gateways, find and see people, see things in the future and find information from the akashic records. Crystals have their own frequencies that are what subtly draw the gazer to the right crystal. I've been using a beautiful Citrine for quite some time now, it has three "spacial" inclusions, that are actually interesting because it looks like it's divided into three worlds. The shamanic "upper" "middle" and "lower" worlds.
So as with any tool in your occult studies, when choosing a crystal ball; go with what calls to you and what feels correct, and have fun with it. You'll get better results ;p