COMET LUCIFER

The story takes place on the planet Gift, where precious crystals called Giftjium are buried in the earth. A young man named Sōgo lives in Garden Indigo, a prosperous mining town. Sougo's hobby is to collect rare crystals. One day, he gets wrapped up in a riot caused by his classmates Kaon, Roman, and Otto, and he loses his way until he finds a lake deep under the ground's mining ruins. There, he meets a mysterious girl with blue hair and red eyes that gaze straight ahead. The story follows the adventure of what happens after this meeting. (Source: Anime News Network) — Description from AniList community (anilist.co).