A crystal ball is a crystal or glass ball device believed to aid clairvoyance. It is sometimes known as a shew stone.

Celtic tribes, known to exist in Britain as early as 2,000 B.C., were unified by a priesthood known as Druids. Druids are one of the earliest known peoples to have used crystals in divination. It's interesting to note that Druid religion had similarities to megalithic religion of an "earlier" Britain, thus it is possible the first use of crystal divination might have come from them.

Later, during central Europe's Medieval Period (500 – 1500 AD), 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, was often used in the divination process. 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 mediums such as water, and are interpreted as meaningful information. The "information" gleaned then is used to make important decisions in one's life.

When the technique of scrying is used with crystals, or any transparent body, it is known as crystallomancy or crystal gazing.

Dr John Dee was a noted British mathematician, astronomer, astrologer, geographer, and consultant to Queen Elizabeth I. He also devoted much of his life to alchemy, divination, and Hermetic philosophy. He was known for his use of crystal balls in his work.

Crystal balls are popular props used in mentalism acts by stage magicians. Such routines, in which the performer answers audience questions by means of various ruses, are known as Crystal Gazing acts. One of the most famous C. G. performers of the 20th century, Claude Alexander, was often billed as "Alexander the Crystal Seer."