The crystal ball is a sphere of quartz or glass used for scrying, and though the technique is the same as with a dark mirror, the object is far more famous, largely for reasons outside religion.
The same technique, a clearer object
The practitioner gazes into the sphere until images are reported to form within it, exactly as with the scrying mirror, so the crystal ball is one member of the family of still surfaces used to occupy the sight and allow inward images to arise. A ball of clear quartz was prized for the purpose, glass serving as a substitute, and the sphere is set on a stand or held so that the gaze can rest in its depth. Nothing in the method distinguishes it in principle from scrying in water, ink or dark glass.
Fame beyond the practice
The crystal ball owes its extraordinary familiarity less to any special standing in esoteric tradition than to its adoption as the emblem of the fortune-teller in popular culture, in fairground, stage and fiction, which fixed it as the very image of divination in the modern imagination. That popular fame has made it the object most people picture when they think of scrying at all, though within the traditions of practice it is one tool among several. It remains in use in modern esoteric and divinatory practice, carrying both its genuine place in the history of scrying and the heavy overlay of popular association that surrounds it.