| A crystal ball is a crystal or glass ball believed by some people to aid in the performance of clairvoyance. It is sometimes known as a shew stone. A body of water, either in a container or on the ground, used for this purpose, is called a scrying pool.
Celtic druids are one of the earliest known peoples to have used crystals in divination. Later, during central Europe's medieval period (AD 500–1500), seers, wizards, sorcerers, psychics, gypsies, fortune tellers, and all other types of diviners also used crystals to "see" into the past, present, or future. Due to its transparent nature, a natural gemstone called beryllium aluminum silicate (beryl) was often used for divination. Scottish Highlanders termed these objects "stones of power." Though early crystal balls were made from beryl, they were later replaced by rock crystal, an even more transparent rock.
The art or process of "seeing" is known as "scrying," whereby images are seen in crystals, or other media such as water, and are interpreted as meaningful information. The "information" gleaned then is used to make important decisions in one's life (i.e. love, marriage, finances, travel, business, etc). By contrast, some professed seers say that they do not actually see images in the crystal itself, but rather that the featureless interior of the stone facilitates them in clearing their mind of distractions so that future truths or events will become known to them. When the technique of scrying is used with crystals, or any transparent body, it is known as crystallomancy or crystal gazing.
Source: Wikipedia, "Crystal ball", available under the CC-BY-SA License.