C chest

The C chest was a treasure chest which was kept by the Doctor within his TARDIS, named for containing a cache of items marked by the letter C. The Doctor carried it as early as his sixth incarnation, when he opened it in the midst of an engagement with the Carrionites. Among the items it contained were a Cyber-helmet, a Cryon pendant, a cabbage leaf, and a copy of Death in the Clouds by Agatha Christie. Curiously, he also found a crystal ball which he was not sure if he had taken at some point in his past or his future. (AUDIO: The Carrionite Curse)

Later, in his tenth incarnation, the Doctor would recover a crystal ball from a trio of Carrionites in 1599, which he would use to contain them after they were defeated. (TV: The Shakespeare Code) He would rummage through this ball as well as a Cyberman chestplate to get to Death in the Clouds with which he demonstrated to Donna Noble that not only did Christie remember her encounter with a Vespiform which that had witnessed, but that her stories would be read well into the far future. (TV: The Unicorn and the Wasp)