A crystal ball is a crystal or glass ball believed by some people to aid in the performance of clairvoyance. A body of water, either in a container or on the ground, used for this purpose, is called a scrying pool.
The art or process of "seeing" is known as "scrying," whereby images are seen in crystals, or other mediums such as water, and are interpreted as meaningful information. The "information" gleaned then is used to make important decisions in one's life