Houseki no Kuni

In the distant future, a new life form called The Lustrous are born. The 28 Lustrous must fight against the Lunarians who want to attack them and turn them into decorations, thus each Lustrous is assigned a role such as a fighter or a medic. Though they hope to fight the Lunarians, Phos is a Lustrous who is given no assignment until the Lustrous' master Adamantine asks them to write an encyclopedia.