In a post-apocalyptic world overrun by Gems, precious crystal beings, who face relentless attacks from the Lunarians, Phosphophyllite, or Phos, a delicate and young Gem, aspires to contribute to the war. Instead, Phos is tasked with creating an encyclopedia due to their fragility. During this reluctant mission, Phos encounters Cinnabar, an intelligent Gem exiled to patrol a desolate island due to their poisonous nature. Witnessing Cinnabar's agony, Phos resolves to find a purpose that would bring joy to both outcast Gems. Enter the realm of Houseki no Kuni, where Phos navigates a journey to find value in their existence and safeguard their kindred Gems.