Ball bearing

Mechanical ball bearings were hard and silver. (PROSE: 64 Carlysle Street) Ball bearings were also a type of sweet, often used to adorn cakes. According to the Eleventh Doctor, ball bearings were his favorite. (COMIC: The Dalek Project) The Tenth Doctor was similarly enthralled and fascinated by their very existence. (TV: Fear Her)

Mechanical ball bearings
Fitz Kreiner once used ball bearings in his musical improvisation, running them up and down his Telecaster electric guitar. (PROSE: The Slow Empire) Heavy rain could be compared to the "clattering down" of ball bearings. (PROSE: How You Get There)

Edible ball bearings
The Fourth Doctor offered K9 a bag of sweets and asked if he would like a ball bearing. K9 asked him not to mock. (TV: The Invasion of Time)

The Fourth Doctor also ate edible ball bearings on Dronid, mentioning that he'd gotten a craving for them. (PROSE: Shada)

The Tenth Doctor thought the commonly seen Earth cupcakes with edible ball bearings were "genius". He said "No other species in the galaxy has ever bothered to make them." Rose Tyler later handed him a cake adorned with ball bearings. (TV: Fear Her)

When a police force were trying to uncover Mamps, Cosmo and Leeb Slitheen from their stone prisons with a want conduit, the Doctor stopped them and arranged that the conduit could only be turned on by thinking about sunflowers and ball bearings. (PROSE: The Slitheen Excursion)

At Hellcombe Factory the Eleventh Doctor used ball-bearings to locate where the magnetic field came from. They stuck to the duplicate of Ralph Hellcombe. (COMIC: The Dalek Project)

Behind the scenes
In the short 1978 sketch Merry Christmas Doctor Who, K9 Mark II tells the Fourth Doctor and Romana I that he wants "enough ball bearing to last out the series" for Christmas.