Being 300 years old, they are the youngest among the gems. Their hardness is 3.5, low compared to most of the other gems.
They are brittle and weak so they are not suited for battle. Despite being clumsy and ineffective they were given the job of completing an encyclopedia.
They want to find new job for Cinnabar, who patrols at night and lives distantly from the other gems.
After being consumed by the sea snail Ventricosus, they gain the ability to understand her speech and decide to follow her into the ocean in hopes of finding a job that might suit Cinnabar.
Ventricosus subsequently offers Phos to the Lunarians in exchange for her brother Aculeasis. Phos loses both of their legs during their capture. Aculeasis escapes the Lunarians and gifts Phos pieces of his shell, which are imbued with agate. Phos' legs are replaced by these, giving them incredible speed.
After the incident at sea, Kongou-sensei decides to cancel Phos' encyclopedia job.
During winter they couldn't sleep and did not join other gems in hibernation. Kongou-sensei allowed them to join Antarcticite in their duties, primarily consisting of upkeep and destroying ice floes. Phos lost both arms when they became distracted by the floes and fell into the water. Phos and Antarcticite search for a new suitable material to replace them, and try out a gold and platinum alloy.
After Antarcticite is taken by Lunarians, Phos is assigned to their previous tasks and helps Kongou-sensei throughout the rest of winter. They have difficulty sleeping, as they have nightmares about Antarcticite being shattered. Phos greatly matures over the winter and mimics some of Antarcticite's looks and mannerisms.
Their new arms can become semi-fluid at will but are very heavy. As a result, Phos loses their previous speed.
In Spring, they temporarily pair up with Bort. While on patrol they are attacked by an unusual type of Lunarian which later turns docile while in Kongou-sensei's presence. Phos overhears Kongou-sensei calling the creature "Shiro" and begins doubting him.
Phos' new goal is to uncover the truth about Kongou-sensei and the Lunarians.
During a fight with the Lunarians, Phos loses their head, leaving their body lifeless. Cairngorm offers Lapis Lazuli's head as a replacement.
A hundred and two years later, Phosphophyllite finally reawakens. They are surprised by their new head, but accepts it easily and think they have become very cute. That night, Phos has a dream in which they meet Lapis Lazuli. Lapis explains that Phos also has a strong connection to their own inclusions, which allows their own personality to persist despite the changes done to their body. They hint at a way to find the truth about Kongo-sensei, but Phos cannot quite figure it out. Upon reawakening, Phos finds that they now have the ability to think and observe things in great detail, presumably much like Lapis did.
With the help of Cairngorm, during the next Sunspot, Phos manages to trick the Lunarians into believing that they have captured the Gem. Phos is taken to the Moon and at first causes a rampage when they learn the Lunarians are able to talk and act normally just like the Gems do. Phos agrees to cooperate with Aechmea and betray Kongo in return for the Lunarians ceasing their attacks on the Gems. Aechmea replaces Phos' left eye with a synthetic pearl one that the Lunarians can use to track their actions. They return to the Land and are given a month to convince the other Gems to leave Kongo.
Phos successfully captures the interest of a few Gems with their stories, but others are suspicious. A month later, Phos returns to the Moon with nearly half the Gems, leaving the remaining ones distraught to the point of shattering.
Upon recovering, Phos stages a nighttime raid on Kongo and the School. It fails, Phos is shattered by Cairngorm and brought back to the moon.
Phos is then given a ship to go down to Earth alone, but is shattered by Bort twice. Their pieces are then scattered and hidden.
Phos is reassembled by Sensei after 220 years, quickly waking up in a feral state. Phos is quickly taken down by some of the Gems but manages to run away. A ship arrives and quickly takes them back to the Moon.
Phosphophyllite returns to Earth with Diamond, Alexandrite, Benitoite, and many other Lunarians to start an attack. After they massacred most of the Gems, including their allies, Phos breaks Kongo-sensei and obtains his right eye, which gives them the ability to pray.
