- You may be looking for the audio short story of the same name.
Crystal balls were objects used to gain unusual insight.
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 TARDIS in perpetuity. (TV: The Unicorn and the Wasp)
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)
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)