Antarcticite, also known as Antarc, is a Gem from the anime Land of the Lustrous who is defined by a unique seasonal existence. Unlike the other characters who remain active year-round, Antarcticite is a crystalline lifeform with a hardness of only 3 on the Mohs scale, making them one of the more fragile Gems. Due to their chemical composition, they exist as a liquid for most of the year, stored in a specialized container within the school during spring, summer, and autumn. Only when the ambient temperature drops significantly during winter do they crystallize into a solid, humanoid form, and the colder the temperature becomes, the harder and sharper they grow.

In terms of appearance, Antarcticite takes a white, crystalline form and typically wears a white uniform that allows them to blend into the snowy landscape, a departure from the dark uniforms worn by other Gems. Their footwear features heels designed like ice skates, facilitating swift movement across the treacherous frozen terrain. When active, they wield a specialized saw-toothed blade, a weapon designed not only for combat but also for the practical task of breaking apart the noisy ice floes that accumulate during the winter season.

Personality-wise, Antarcticite presents a calm, composed, and businesslike demeanor. They approach their solitary winter duties with the discipline and efficiency of a soldier, taking on the responsibility of guarding the hibernating Gems, clearing snow, shattering ice, and performing general maintenance. This stern and serious exterior, however, masks a deeper, more affectionate nature. Antarcticite holds a profound love and respect for the enigmatic Master Kongo, the leader of the Gems. As one of the few characters who spends extended one-on-one time with him during the winter, they see a more vulnerable and tired side of him that others do not. In these private moments, Antarcticite drops their formal act, revealing a desire for his approval and even secretly craving physical affection, such as hugs, from him. They are deeply aware that Kongo and the Gems are different lifeforms but continue to serve him out of pure devotion.

Antarcticite's central role in the story is intrinsically tied to the protagonist, Phosphophyllite, commonly known as Phos. Initially, Antarcticite shares the common perception that Phos is useless and refuses to let them assist with winter patrols. However, they are moved by Phos’s genuine desire to find a purpose and decide to take on the reluctant mentor role. Unlike other Gems who coddle or give up on Phos, Antarcticite pushes them with strict, relentless training. They do not offer help when Phos struggles but wait for them to stand up on their own, treating them with a form of respect that challenges Phos to change.

This relationship is pivotal, marked by both guidance and tragedy. Antarcticite blames themselves for Phos losing their arms during a training accident, citing their own lack of teamwork skills as the cause rather than blaming Phos. This self-recrimination highlights a key aspect of their motivation: a desire to prove their worth and protect their charges. The climax of their development occurs during a battle with the Lunarians at the Shore of Nascency. While trying to free Phos from a binding alloy and retrieve new materials for their arms, Antarcticite is caught off-guard by a second wave of moon people. In their final moments, they entrust the winter duties to Phos with the poignant request to take care of things so that Master Kongo will not be lonely. They are then shattered and taken to the moon. Due to their low hardness, their particles were partially scattered or evaporated, making them impossible for even the Lunarians to restore.

Notable abilities include their combat proficiency, which is highly dependent on the environment. While fragile in warmer conditions, their strength increases dramatically in sub-zero temperatures, making them the most effective fighter during winter. They are skilled in using their large ice pick-like sawtooth blade for both offensive strikes against the Lunarians and for the environmental task of clearing ice. Furthermore, Antarcticite possesses the unique biological trait of remaining liquid in a dormant state during the off-season, a physical representation of their seasonal role as the sole guardian of the winter landscape. Their capture serves as a major turning point for Phos, instilling a deep sense of guilt and a drive for power that fundamentally alters the course of the narrative.