Kalafina (カラフィナ) is a Japanese band formed by the composer Kajiura Yuki (梶浦由記) in 2007. As the group started, it performed many theme songs for the anime Kara no Kyoukai (空の境界).
Here are some PV's from their official youtube channel. They have beautiful voices, it is definitely worth listening to!
Also here is the link to their official website! -> KALAFINA OFFICIAL WEBSITE