Cat brooch

A cat brooch or cat badge was a form of brooch shaped like a cat. The Sixth Doctor possessed a variety of cat brooches, all of which he wore on the left lapel of his coat.

The Doctor occasionally touched, tapped, or rubbed his cat brooch for good luck just before doing something particularly risky. (TV: The Twin Dilemma, Attack of the Cybermen, Vengeance on Varos, PROSE: Retribution, Time Wake)

When Peri Brown was put on trial for a murder she didn't commit, the Doctor proved her innocence in part by using his cat brooch to collect a blood sample from the real killer. (COMIC: The Forgotten)

When the Doctor and Maisie were tied to chairs in a cave, Evelyn Smythe sharpened the Doctor's cat brooch on the rock wall and used it to cut them free. (AUDIO: A Town Called Fortune)

One of the cat brooches appeared to change its expression to match the Doctor's mood. (WC: Real Time)

Insisting that the Doctor return his coat in exchange for his own, Jack Harkness said that he would have put cat badges on it. (AUDIO: Piece of Mind)

Behind the scenes

 * One original idea for the brooch was that it would change its expression to match the mood of the current situation, for example, frightened cats when the Doctor was scared. Ultimately, this idea was dropped, though it would be subtly implemented in the illustrated webcast Real Time.
 * Many fans sent cat brooches to Colin Baker; he eventually accumulated a large collection, and claims to have worn every single one. (DCOM: Attack of the Cybermen) When Baker reprised the role of the Doctor in the 1989 stage play The Ultimate Adventure, his costume contained a different brooch from his collection for each performance.