Quartz no Oukoku

A Kingdom of Quartz is the story of elegant creatures with magnificent wings and peerless strength who, by the Goddess's will, protect the Kingdom of Quartz from infiltration by demonic forces: These are the heroes known as Angels. A little girl named Blue, who was born with mysterious black wings, spends every waking moment dreaming of becoming an Angel herself. Then, one day, Blue meets a prince, and the cogs of her fate twist and heave, enmeshing her in tragedy and adventure…

(Source: Anime News Network)