Kaito royale (2011)

Thieves lurk at a high society party hosted by a jeweller. Their target is an alexandrite ring called “Alexander's tear” and said to be the the best jewel among jewels. Kamimura Rei has been recruited by Nishizaki, the butler of a mysterious woman called Kitaba Yoko, to steal the ring. Katagiri Karen and Kishihara Taiga have each been recruited for the same purpose too, and they plot to get close to the target. Rei, who prefers to play the lone wolf, had accepted the job because Nishizaki had hinted at knowing something about lost memory. Neither Rei, Karen, Taiga nor another thief are aware of each other. And so, this leads to an all out demonstration of their ability to cheat and steal what has been stolen. The tragic history of the ring which had caused its owner to die and a shadowy evil organisation pose challenges to them. ~ JDrama Weblog