Alexandrite is a gemstone character from the series who holds a hardness rating of 8.5 on the Mohs scale, making them notably resilient and capable in combat. Among the community of Lustrous, Alexandrite is distinguished by a deep and all-consuming obsession with the Lunarians, the moon-dwelling adversaries who constantly attack them. Their primary assigned duty is to collect, analyze, and catalog data on these enemies, a task they pursue with exceptional dedication. This role establishes Alexandrite as the leading expert on Lunarian behavior and tactics, and other gems frequently rely on their extensive knowledge for strategic planning.

Alexandrite's personality is defined by a driven, research-oriented nature that borders on the obsessive. This intense focus, however, is not born from simple curiosity but from a deep personal trauma. Long ago, Alexandrite lost their original partner, a gem named Chrysoberyl, who was taken by the Lunarians. To cope with this loss and ensure they never forget the pain and hatred it caused, Alexandrite channels all their energy into studying their enemies. The stated goal is to maintain a fresh, undiluted feeling of hatred towards the Lunarians, using research as a means to keep that emotional wound from healing. This history makes them one of the older gems in the school, though their exact age is unknown.

In the story, Alexandrite serves as a crucial source of information, particularly for the protagonist Phosphophyllite. When Phos begins to question the nature of their world and the true identity of their leader, Kongo, they seek out Alexandrite for a comprehensive re-education on the Lunarians. This study session, which lasts an entire night, highlights Alexandrite's rigorous and almost manic teaching methods, as they drill Phos relentlessly on Lunarian lore. Beyond their research role, Alexandrite is also a frontline fighter, but their combat style is uniquely volatile. Encountering a Lunarian triggers a specific condition in Alexandrite, sending them into a berserk battle frenzy known as their red state. In this mode, their hair color shifts, and they become a silent, single-minded force of destruction focused solely on eliminating the enemy, often losing consciousness after the battle concludes. Because of this uncontrollable reaction, Kongo has forbidden Alexandrite from directly engaging with Lunarians unless absolutely necessary.

Alexandrite's key relationships are largely defined by their past and their singular obsession. The most significant figure in their backstory is the lost Chrysoberyl, whose absence dictates Alexandrite's present actions. Their interactions with other gems, such as the weary Yellow Diamond who is seen resting near them, are often secondary to their work. Later in the narrative, Alexandrite undergoes significant development, particularly after events shift to the moon. It is revealed that the doctors on the moon possess the technology to modify Alexandrite's body, allowing them to remain permanently in their powerful red, battle-frenzy state. Alexandrite expresses a desire for this permanent transformation and even asks to accompany Phos back to Earth in this enhanced form, indicating a willingness to embrace their destructive side fully. This development shows a character willing to sacrifice their stability and consciousness for the sake of power and the eradication of their hated foes. Their notable abilities, therefore, include not only their extensive analytical mind but also this unique physical transformation that drastically increases their combat effectiveness, making them one of the most dangerous and unpredictable fighters among the Lustrous.