Phosphophyllite

Phosphophyllite, or Phos for short, is the main protagonist of the Houseki no Kuni anime and manga.

Phos is the youngest Gem when the story began. They're weak, brittle - not suited for battle - and their clumsiness makes them difficult to maintain any other roles. Phos has always wanted to be useful.

Phos' initial job was to make a natural history encyclopedia which they progressed very little on. They promised to help Cinnabar find a job suited for them as well.

Phos so far is the only Gem who has had drastic body and personality changes.

Appearance
Phosphophyllite has mint-green features known by the Gemsand the Admirabilis as the Moon People's favorite color. Their hair is short, originally just beneath the ears, but cut shorter in later chapters to imitate Antarcticite's hairstyle. Over the course of the story, parts of their body are replaced with other gems and metals. They lose their original legs when Ventricosusattempts to trade them with the Moon People for her brother. After a failed negotiation with the Moon People, Ventricosus changes her mind and gifts them parts of her brother's shell with agate inside as a replacement, which gave them mobility unbeknownst to any Gems.

After losing their arms during winter duties, they were replaced with gold and platinum alloy, which made their arms gold in color and Phos larger in size. Their new arms can shift form and become semi-fluid at will. They also use their alloy to give their shoes heels, likely inspired by Antarcticite. From this point on, Phos carries Antarcticite's blade and uses it during patrol and battles.

After losing their head to the Moon People, it was replaced with Lapis Lazuli's head instead. They wear Lapis' hair in a high ponytail when Cairngorm asks then to change it due to them appearing too similar to Lapis. Soon after, it gets cut short by an arrow during an attack. On the Moon, Aechmea replaces Phos' left eye with a synthetic pearl replacement that emits light in the dark.

They wear a standard winter uniform and black shoes without socks, occasionally accompanied by long black gloves.

Personality
Phosphophyllite is unique in being the only Gem to exhibit vast personality changes over time. Phos was initially clumsy, spoiled, and had a flexible way of thinking. They talked big about themself, but were actually aware that they were useless. Prior to their many changes, they hid their frustrations by acting naggy and spoiled. They were the only Gem who talked back at Kongo-sensei. Despite their overall lack of ability, they were eager to fight because they loved Kongo-sensei and wanted to help him just as all the other Gems do. Even though Phos desired to join the battle, the first job they got after 300 years of unemployment was being a scholar and making a natural history encyclopedia. Phos disliked this job from the beginning and had a very difficult time completing it even with the aid of other Gems. They didn’t like hard work but were willing to do it in order to be useful. Phos had a habit of saying, "I'll do better next time", although they didn’t always accomplish that.

Phosphophyllite tends to do reckless things, and has on many occasions disobeyed Kongo-sensei, more so as time went on. An example of this is when she followed Ventriscous into the ocean. Because of the way the Gems view life, Phos accepted Ventriscous’ reason for her betrayal even though they could have been captured by the Moon People.

Phos' personality began to change after Antarcticite’s abduction, which was also around the same time Phos lost their original arms and had them replaced with a gold-platinum alloy. They were traumatized and began to hallucinate and have nightmares about the event. Their new arms gave them strength and great ability though, especially in combat where Phos became easily able to defend and attack the Moon People. This new-found strength caused them to become more confident, humble, and mellow. But they also became melancholic and solemn, no longer possessing the happy, energetic, and carefree attitude they had before.

After the incident with Shiro, Phos overheard Kongo talking with the Moon People and began to distrust and question Kongo-sensei. After their original head was taken by the Moon People and replaced with Lapis Lazuli's, they become more level-headed, cautious, observant, and manipulative, just as Lapis had been. They now focus more on their own goals and the safety of the others Gems over their concern for Kongo-sensei.

They use a masculine first-person pronoun boku.

Trivia

 * Phosphophyllite is mentioned to have a strong connection with their own inclusions, which allow their memories and personality to be kept more intact even if they lose pieces of themselves.
 * Since Gems are mostly inorganic matter, change isn't something that normally happens to them, but Phosphophyllite has undergone many physical changes throughout the story, so much so that more than half of their original body has been replaced by new parts. The series toys with the idea of a Ship of Theseus, a philosophical thought experiment that asks whether an object is still the same if parts of it are placed bit by bit until eventually none of the original parts remain.
 * Ironically, in the beginning, it was suggested by Diamond that Phos change themselves to improve, though Diamond meant this more in a behavioral way.
 * From chapter 21 onwards, the agate pattern on their legs was masked by the powder because Phos wanted to stand out less.
 * The collaboration with TASAKI features a Phosphophyllite necklace that has 18K white gold, a South Sea pearl on the left, and a teardrop shaped Phosphophyllite gem on the right.
 * At some point, Phos had an arm made of gold-platinum alloy attached to their body to replace their lost arms. Since it was too heavy for their brittle body to lift, the alloy spread and wrapped around Phos' body like a net or lattice, strengthening it. This may be a reference to Kintsugi, the Japanese technique of repairing broken pottery with lacquer that has been mixed with gold, silver, or platinum. This technique is used to embrace the flaws of the broken object rather than disguising it.
 * Their debut appearence resembles that of Nino Nijiro from Idol Time Pripara.