A marble was a small round toy.
A Dogon's sixth eye resembled a blue marble. (TV: Random Shoes)
On Go-Ray, the First Doctor, John and Gillian escaped from some Go-Rays when the Doctor scattered John's marbles on the ground, causing the Go-Rays to slip and fall. (COMIC: Enter: The Go-Ray)
When the Second Doctor visited the Panjistri homeworld, he had a bag of glass marbles in his pocket. (PROSE: Timewyrm: Apocalypse)
Amy and Rory escaped from a Sontaran ship in 1605 by firing marbles from a slingshot at the Sontarans' probic vents. (GAME: The Gunpowder Plot)
While aboard the HMS Thunder in 1941, the Seventh Doctor removed several marbles from his pocket. (AUDIO: Dark Convoy