This Boy Suffers from Crystallization

The story revolves around high school boy Ayumu Banri, who has an illness that makes him start to turn to stone when he's stressed. Since he couldn't fit in with his class, after repeating a year, he declared that he wanted to have a "Sparkly youth" as well. To do this, he changed his hair and clothes to be more fashionable, and constantly checked for popular topics to stay in the know. In the midst of Ayumu's life of making a facade, his homeroom teacher and stone-loving geology teacher Kouya Onihara says that Ayumu's stone transformation is beautiful. Ayumu begins to become attracted to Kouya, who starts to give him advice.

(Source: Anime News Network)