A crucial part of the Chameleon Arch looks like a fob watch.
Chameleon Arch fob watches once held the Time Lord essences of both the Tenth Doctor and the Master. Such watches were recognisable, as Martha Jones learned, by distinctive Gallifreyan messages on the obverse. (TV: Human Nature, The Family of Blood, Utopia)
The fob watch carried by the Seventh Doctor was the one used by the First Doctor. (PROSE: All-Consuming Fire)
The Second Doctor used a fob watch to hypnotise Vana after she exited the Kroton spaceship. (TV: The Krotons)
Once, the Fourth Doctor used his fob watch for hypnotic suggestion. (TV: The Ribos Operation)
Zoe Heriot asked the Sixth Doctor for his fob watch so she could hypnotise herself to keep a Cyber-Planner out of her head. (AUDIO: Last of the Cybermen)
The Eighth Doctor wore an outfit consisting of a deep green velvet trench coat and a bronze-grey waistcoat with a fob watch. (COMIC: The Pictures of Josephine Day, TV: The Night of the Doctor)
The Eleventh Doctor began to regularly wear a watch chain after going into retirement and continued wearing it after his return, when he changed his clothing style. (TV: The Snowmen, The Bells of Saint John) It was generally a gold fob watch fastened to his waistcoats. (TV: The Day of the Doctor, The Time of the Doctor) The Eleventh Doctor used a fob watch to lure Lepus Warriors through a warp gate. (COMIC: The Hat Trick)
The Ninth Doctor once gifted a modified Chameleon Arch to his friend Plex, with the purpose of creating a clone race. It would be used in his twelfth incarnation as proof to said clone race that he was an old friend of Plex's. (COMIC: The Promise)
On meeting Jackson Lake and seeing his fob watch, the Tenth Doctor thought that Jackson might be an incarnation of himself. When the Doctor turned the watch over, however, it didn't bear any Gallifreyan script and was revealed as an ordinary fob watch. (TV: The Next Doctor)
* Each watch comes with a velveteen pouch, small tool for changing the battery and a couple of extra batteries!