Diamond, often called Dia, is a Gem who primarily serves on patrol duty within the hierarchy. While possessing significant offensive power, Diamond is noted for having relatively low durability compared to other Gems, making them prone to fracturing in combat. They are highly regarded for their elegant and aesthetically striking appearance, which often draws comparisons to a fashion model.

On the surface, Diamond comes across as kind, compassionate, and serene, maintaining a calm demeanor even in unfavorable battle conditions. However, this outward tranquility conceals deeper feelings of inadequacy and frustration regarding their own physical form. Diamond is acutely aware of their vulnerability to fracturing and harbors a strong desire to overcome this weakness. This frustration is closely tied to their relationship with Bort, a junior Gem for whom Diamond has high regard but deeply dislikes being compared to.

In their role within the story, Diamond demonstrates a keen intuition for the emotions and dynamics of the other Gems, though Cinnabar has suggested that Diamond tends to romanticize every interaction. Despite their generally amiable nature, Diamond displays remarkable tenacity in battle, often persisting to the point of self-destructive recklessness. This behavior is noted by Bort, who views Diamond's combat style as alarmingly dangerous. This tenacity symbolically reflects the physical hardness of a diamond, showing that their unyielding nature is a key part of their character. Diamond's personality also carries a romantic and sweet disposition, which is mirrored in their narrative arc and interactions. A unique detail of their appearance is having a customized component of their attire, which metaphorically underscores their individuality and rarity.