Yuuzora ni Ukabu Tsuki

Yuuri Nishi is a rookie at an atelier, struggling to master his craft. While learning sewing from his admired senior, Mitsuru Sumizuki, he quietly worries that he may never get close to him. Then Yuuri discovers that Mitsuru suffers from a rare condition known as "jewel disease," a mysterious ailment that turns emotional pain into gemstones. He realizes that Mitsuru's distant behavior has been shaped by this secret. Determined to protect Mitsuru's heart, Yuuri cares for him gently, and gradually, Mitsuru allows his lonely heart to draw near. But Mitsuru harbors another secret, one that may make it impossible for him to ever fall in love with Yuuri...

(Source: Kadokawa, translated)

Note: Includes one extra chapter.