Chalk
From Tardis Wiki, the free Doctor Who reference
Chalk was a drawing and writing tool.
A Doctor's item
A piece of chalk was often produced from the Doctor as one of his items, since his first incarnation (TV: The Sensorites) to the eleventh. (TV: Hide)
In Castrovalva, the Fifth Doctor used it to draw a map of the town. (TV: Castrovalva) When he visited Sair, he found chalk drawings on the floor of his room, and traced them with a piece of chalk to reveal their shape. (PROSE: Fascination)
The Seventh Doctor gave a piece of chalk to Ace in order to draw a protective circle for her, Shou Yuing and Excalibur from Morgaine. (TV: Battlefield)
Other uses
The scientist Rubeish used chalk to write equations over the Third Doctor's TARDIS (TV: The Time Warrior)
