Comet Lucifer (2015) 6 Seasons

On the planet Gift there are precious crystals called Giftjium buried in the earth. A young man named Sougo Amagi lives in Garden Indigo, a prosperous mining town. Sougo, whose hobby is collecting rare crystals, one day becomes involved in a dispute between classmates Kaon, Roman and Otto. He wanders deep into the ruins of a mine and discovers an underground lake. There, he meets a mysterious girl named Felia with blue hair and red eyes. Who is this girl, and what will their meeting bring?