The Hundred-Year Crystal Catalogue(2013)

Hyakunen Kesshou Mokuroku

Bentt, a traveling scholar, comes across a young man named Iris in an abandoned mine. Iris eats crystals, has eyes that sparkle like rainbows, and is the last surviving member of a race known as the Diamonds of the Desert. Setting out on a journey together, the two begin to slowly open up their hearts to each other. However...

(Source: Printemps Publishing)

Note: Includes one extra chapter.