Chalk

Chalk was a drawing and writing tool.

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)

The scientist Rubeish used chalk to write equations over the Third Doctor's TARDIS (TV: The Time Warrior)

The Twelfth Doctor found a piece of chalk in his bedroom, in the house of the Paternoster Gang, and used it to write equations on the floor. (TV: Deep Breath) Later he kept on writing on a blackboard in the TARDIS control room. (TV: Into the Dalek onwards)