"Land of the Lustrous" is set in a distant future where Earth has been ravaged by meteor impacts, leaving only a single immortal humanoid species known as the Gems to inhabit the world. These Gems are genderless, crystalline beings who possess unique physical and mental traits based on the minerals they are composed of. They live in a tranquil yet fragile existence, constantly under threat from the Lunarians, mysterious moon-dwelling beings who seek to harvest the Gems for their beauty and use them as ornaments or weapons.

The protagonist, Phosphophyllite, or Phos, is a young and fragile Gem with a hardness of only 3.5, making them ill-suited for combat. Initially, Phos is assigned mundane tasks due to their brittleness, but their desire to contribute meaningfully leads them to seek out new roles. This journey of self-discovery begins when Phos is tasked with creating a natural history encyclopedia, a project that introduces them to Cinnabar, a reclusive Gem whose mercury-based body poisons everything it touches. The two form a complex relationship as Phos strives to find a purpose for Cinnabar, who feels isolated and useless.

As the story progresses, Phos undergoes significant physical and emotional transformations. After losing their original body in a battle with the Lunarians, Phos is reconstructed with gold and other materials, gaining new abilities but also experiencing a shift in personality and priorities. These changes create tension with the other Gems, particularly their mentor, Kongo-sensei, a mysterious figure who protects the Gems but harbors secrets about their origins and the true nature of the Lunarians.

The narrative explores themes of identity, purpose, and the cost of change as Phos becomes increasingly determined to uncover the truth about the Gems' existence and the Lunarians' motives. The series delves into the psychological toll of immortality, the struggle for self-worth, and the consequences of seeking answers in a world where knowledge is both a weapon and a burden. Through its intricate character dynamics and philosophical undertones, "Land of the Lustrous" presents a thought-provoking and emotionally resonant story set in a visually striking and imaginative world.