YouTuber Vikkstar123 has made a pretty great video which shows the mysterious Diamond Camo and confirms how it is unlocked. In order to unlock it, you must “Complete all camo unlocks for all specials”.
SOURCE: Vikkstar123 Channel
Source Article from http://www.charlieintel.com/2012/11/16/black-ops-2-secret-diamond-camo-and-how-to-unlock/