Guardian crystal

After forming a contract, a crystal was given to agents of the Black Guardian to keep in contact with. The crystal would be tuned to their mental wavelengths, whispering instructions to the agent and glowing bright. Like the Key to Time, every particle of it existed at every point in time. (PROSE: Terminus, The Well-Mannered War)

Turlough's crystal was able to stick to his skin and inflict pain. It cracked during a release of time differential (TV: Mawdryn Undead) but later repaired itself. It blackened completely when the Black Guardian was temporarily destroyed by Enlightenment, terminating the contract. (TV: Enlightenment)