- You may be looking for the audio short story of the same name.
Crystal balls were objects used to gain unusual insight.
Olive Hawthorne kept a crystal ball (or "reticule", as she referred to it) in her handbag, which she used to knock unconscious one of the Morris dancers who served the Master, after the hypnotised villager attacked John Benton with a stave in The Cloven Hoof. (TV: The Dæmons)
On one occasion, three Carrionites used a crystal ball to view and possibly influence remote events through a form of verbal science that appeared to 16th century humans as "magical". They were eventually imprisoned within it by the Tenth Doctor, (TV: The Shakespeare Code) who retained the ball within the C chest in the TARDIS in perpetuity. (TV: The Unicorn and the Wasp) Much earlier, the Sixth Doctor, in the midst of an engagement with the Carrionites, curiously found a crystal ball within the chest which he was not sure if he had taken at some point in his past or his future. (AUDIO: The Carrionite Curse)
On another occasion, a fortune teller on Manussa told Tegan Jovanka that she used a crystal ball to pretend to see into the future. Tegan, who was at that time possessed by the Mara, showed her an image of a snake's skull in the crystal ball. (TV: Snakedance)
Morgana used a crystal ball in her fortune-telling act. The Gods of Ragnarok communicated with the Psychic Circus through the ball, and showed them images of their foes' actions. The ball exploded when the Seventh Doctor destroyed the Gods. (TV: The Greatest Show in the Galaxy)
Jared Khan used a crystal ball while making contact with the Charrl, but did not actually need it; its sole purpose was to impress the other members of the Brotherhood of the New Dawn. (PROSE: Birthright)
One crystal ball was an intelligent crystalline structure which could create a virtual world where it consumed the weakness and despair of its victims' minds inside a computer program. According to Diamanda, it could speak to her, and was found in an antique shop. The Twelfth Doctor believed it probably came to Earth when a spaceship crashed, and it was carved into a globe. Diamanda was consumed completely when the program created a world resembling a theatre with death traps, but the Doctor, Clara Oswald and Harry Houdini were able to escape, destroying the crystal ball. (COMIC: Theatre of the Mind)