A crystal ball is a crystal or glass ball 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 crystal balls 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, gypsie, fortune tellers, and all other types of diviners also used crystal balls to "see" into the past, present, or future.