Nothing is credited with the magical power of divination more than the humble Crystal Ball. It is mentioned in the book of revelations and referred to in the legend of King Arthur, who sought out the advice and prophecies of Merlin the Magician via his crystal ball..
Prominent in the middle ages, Scryers (fortune tellers) revered their crystal balls, often passing them on to their apprentices. A tool for reaching and communicating with the divine, indeed crystals have always been used in communications ~ just as they are today in radios, telephones and computers.

Possibly the reason crystal balls became important tools is because of their inherent characteristics of transparency and symmetry. Additionally it is claimed the energetic fields of crystals and gemstones influence what the scryer sees on a very subtle and often esoteric level.

Warning!
It may seem self-evident but it is worth noting that because of the optical nature of crystal balls, if left in direct sunlight there is a slight possible fire hazard.
For example take care when displaying in windows.