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Gems / 宝石
Leader Kongo-sensei (Earth Gems)
Phosphophyllite (Moon Gems)

Gems (宝石, Hōseki) are an immortal, humanoid species made of inorganic minerals that inhabit the land of the planet. There are 28 gems on the island at the start of the story.


The Gems take the forms of humans, but with bodies made of polished gemstone that differ depending on their material composition. Most of them have hair, eyes, and nails the same color as their gemstone bodies and white skin made from powder they apply to protect from sea breeze and harsh sunlight. While their hair may look solid, it can be manipulated similarly to human hair (cut, tied, braided, etc). Apart from the adult Kongo-sensei, their appearances take the form of androgynous young teens standing at 150cm on average.

The Gems are born at the Shore of Nascency every 100 to 1,000 years on average, where crystals rose up from the sea over eons. Some of these crystals fully develop into an angular humanoid form (though lacking eyes) and become a Gem, while others never reach that stage and simply fall back to earth as inanimate crystals. Kongo corrects each Gem with a chisel and grants them eyes after they are born on the Shore of Nascency, removing asymmetries to make all the gems equal. The Gems lack any sexual characteristics and even navels, making them completely sexless. Their outer body functions much like a human's, and they appear to be able to use all five senses, though they are numb to temperature. They can determine another Gem's hardness by touch.

A Ruby that failed to fully form into a Gem.

The same gem falling onto the beach.

Each Gem is differentiated by their hardness and toughness. Some Gems can be very delicate, making them unsuited for battle, while others are very hard and durable. They use a scale of hardness that is equivalent to the Mohs scale, which is based on the ability of one mineral to scratch another, with 1 being the lowest and 10 being the highest. Toughness or brittleness is the measure of how easily a Gem breaks and is more complicated to determine. For example, Diamond has the highest hardness of 10, but is second-class in terms of toughness. Gems like Jade can have an average hardness of 7, but have excellent toughness. One way to sum it up is to consider hardness the level in which a Gem can break other objects, while toughness is the level in which they themselves can resist being broken.

Gems carry microscopic organisms called "inclusions" (インクルージョン) in their bodies, which energize them by absorbing light and allow them to move. Due to this mechanism, Gems need sunlight to survive, forcing them to sleep at night and hibernate in winter. Furthermore, inclusions allow Gems to be put back together when shattered, making them technically immortal. The oldest known Gem besides Kongo-sensei is Yellow Diamond at 3,597 years old, with Phosphophyllite as the youngest Gem at the start of the series, being only 300 years old.

Phosphophyllite after being scraped.

Their memories (and presumably their personalities) are stored within the gemstone that makes up their bodies. As such, losing any part of their body will result in some memory loss. Their lost body parts can be replaced with other materials as long as they are compatible, usually in the form of similar mineral composition and hardness. The hair of the Gems, however, contains little-to-no inclusions, so losing it is not a serious problem.

They can speak underwater but their powder skin can wash off, requiring reapplication. Salt-water (as in the sea and the school's pond) can damage some gemstones so anti-salt resin is required. The waves are also dangerous since the pressure can shatter their body. Acid and heat can also be dangerous to Gems, as demonstrated when a giant slug digested one of them. In this case, the Gem who was digested was incorporated into the slug's shell as fragments and was able to be revived. It is unknown if Gems can be truly dissolved and thus killed. The poison from Cinnabar can also harm Gems as any part of their body that comes in contact with it will be tainted and blocked from the light, preventing the inclusions inside from feeding. These parts must be scraped off the body.

In critical situations, some Gems can fix themselves without the assistance of resident doctor Rutile if their fragments are mostly together and in one place, though they don't do as good a job as Rutile. It is shown that they still can speak when they're broken as long as they are mostly intact, but will lose consciousness when left in mere fragments. Intense negative emotions can also cause them to suddenly fracture. Being broken or melted is described as feeling very sleepy and being paralyzed. Being hit by another Gem appears to make them uncomfortable, as shown when the Amethyst twins bump heads constantly, much to Phosphophyllite and Yellow Diamond's discomfort.


As told by Kongo-sensei

According to him, the planet was broken after being visited by a meteor for the sixth time. It gave birth to six moons, leaving it an emaciated shadow of its former self. There was nothing left but a single beach, so all life fled to the ocean. Some creatures who had flourished when the moon was still one were too slow in their escape and sunk into the seas. They were eaten by tiny creatures on the ocean's floor, turned into inanimate objects, crystallized over the ages, and rose to the surface at the Shore of Nascency. In rare instances, this leads to a Gem successfully taking form.

The Gems have to defend themselves against the endless number of hunters from the Moon - the Lunarians - who abduct the Gems and, apparently, use them as ornaments. At the start of the story, no Gem has ever returned from the Moon.

As told by Ventricosus

According to the Admirabilis' legend, there once were creatures called humans who lived on and preserved the land until it broke five times. When it broke the sixth time, they entered the seas. It's said that there, they split into three parts; the spirit, the flesh, and the bone (taken less literally, they evolved into three different species and lived on). The Admirabilis are said to be the flesh, inheriting the ability to expand and pass down their knowledge; The Gems the bone, returning to the land after having made a contract with another living species and gaining the ability to live for millennia; and the Lunarians as the spirit, forever wandering, attempting to regain its flesh and bone.


The Gems use male pronouns (彼(kare)/he; お兄様(onii-sama)/older brother; 弟(otouto)/younger brother) to refer to each other. While most of them also use masculine forms of "I" (僕/boku, 俺/ore), their personal speech patterns vary greatly between masculine and feminine. Even so, they don't seem to be aware of gender and/or sex characteristics, as seen when Phosphophyllite asks Ventricosus about her breasts, referring to them as "water bags". That instance aside, their understanding of gender is rarely brought up or explained within the work. Whether they see themselves as male or genderless is left unconfirmed, but is implied to be the latter. Most call each other with various nicknames, but all call Kongo "Sensei" (Teacher). 

The gems 2.jpg

The Gems live in a spacious school building with a large bell that they use as a signal. The school uses jellyfish as lighting instead of electricity-based technology or fire, neither of which have been shown being used by Gems. They also use traditional tools for hand-spinning textiles and crafting. They're able to produce textiles by themselves, craft tools, make papers and pencils, and have developed science in medicine and herbs, though largely gem-related. Kongo-sensei teaches the young Gems anything they need to know, though the school does not seem to hold any organized classes, teaching the gems in one-on-one settings away from the others.

Each Gem is entrusted with a role or roles they're suited for, though almost all of them are fighters and have patrol duties during the day. The different roles shown so far include patrolling, health and medicine, crafting, smithing, tailoring, and strategizing and planning. They each have their own dorm rooms to sleep in at night and hibernate in one big room in the winter. In their free time, they may play card games, which were either taught to them by Kongo-sensei or created themselves.

Every time the Moon People come, the Gems must ring the bell to alert everyone of the situation (for instance, six rings tells the Gems to stand by and await further orders). Patrol teams usually consist of two Gems, which leaves one to fight and the other to keep watch or run back and alert Kongo-sensei in the case of an encounter. Usually, after the bell is rung, all the Gems come together with him, though he will always disperse the Moon People with his powers alone. Every morning they have an assembly, which is especially important for those patrolling that day.

Most Gems use the international/English name of gems for their own names, but Japanese is used for 'Kongo', meaning Adamant, and 'Shinsha' for Cinnabar.

Except for Kongo-sensei, who wears a Buddhist monk kimono, the Gems wear black and white uniforms sewn by Red Beryl for their day-to-day lives. Their winter uniform is a high-collar romper-style suit and ties with shorts and puffy sleeves, while their summer uniform is a loose dress shirt with a black tie and culottes. At night they wear white kimono-style pajamas, with more elaborate gowns worn during winter hibernation. The uniforms are largely consistent, changing only slightly every few centuries. Red Beryl, the gem in charge of tailoring, likes to make things feminine and frilly, decorating the insides of the winter uniforms with bright floral patterns.

High-hardness gems wear specific articles of clothing at all times: diamonds wear thigh-high socks and long gloves, while corundum gems (sapphires/rubies) wear shorter socks that go just over the knee. Even jade-type (nephrites/jadeites) gems wear special knee socks. The gloves are a requirement so as to not damage softer Gems by accident, with black gloves carried by all Gems during winter and white ones in summer. The different socks are said to be a way to identify harder gemstones from a distance.

though Red Beryl switches it up slightly every few centuries. High-hardness gems wear specific articles of clothing at all times: diamonds wear thigh-high socks and long gloves, while corundum gems wear shorter socks that go just over the knee. Their pajamas and winter hibernation clothes share similarities with kimono. Red Beryl, who in charge of tailoring, tends to make things feminine and frilly.

Due to their unique structure, they have no concept of true death and tend to cherish short-lived things like paper and wooden tools. They tend to think being mortal like other organisms is an inconvenience, and some (like Bort) believe that Gems are superior. They consider it unethical for the Moon People to kidnap their brothers and use them as tools. Since they are technically immortal, there is a chance for kidnapped Gems to be returned from the Moon, even if it means doing so piece by piece. Some gems do not show too much emotion over the loss of their brothers, perhaps due to the fact that they can be returned. Others may become stricken and obsessed over their kidnapped siblings, especially if they were involved in the raid that caused it to happen and blame themselves.

Excluding Kongo-sensei, the Gems have no idea about the existence of humans. Kongo-sensei occasionally refers to human characteristics, such as Phosphophyllite's tears, as "the dregs of an ancient species", though never uses the word "humans". The Gems don't seem to show much curiosity about things beyond their own tasks. That being said, they seem to have a fondness for flowers. They gather bouquets during sensual moments, have hand-made flower bowls, and grow a lot of various plant and flower species beneath the school roof, presumably growing and caring for them. When Phos got golden arms, they shaped the gold into flowers to show the other Gems that the hands are not dangerous, ineffective as it was.


They are caring towards each other and will go to great lengths to defend each other against the Lunarians, even sacrificing themselves if necessary. They all unconditionally love their only teacher and are commonly seen doting over him. Kongo-sensei essentially acts as a parental figure towards the Gems, having raised and educated every single one of them from birth. Despite his stoic demeanor, Kongo loves the Gems as well and will occasionally give them affection as if they were his own children. The Gems have been seen requesting hugs, trying to win his attention, or displaying jealousy over other Gems who receive it more.

List of Gems

Gems on Earth at the beginning of the story

At the story's introduction, Kongo-sensei states that there are 28 Gems. This was later stated by Cinnabar as well, though only 27 individuals Gems have been seen, even after including Kongo-sensei himself, the Amethyst twins, and Ghost Quartz's other personality Cairngorm. It is currently unknown who the remaining Gem(s) are.

Gems born after the beginning of the story

Gems taken to the Moon before the beginning of the story

List of Gem Factions

Earth Gems

Gems who stayed on Earth following the events of Chapter 62.

Moon Gems

Gems who defected to the Moon during the events of Chapter 62.


  • In mineralogy, an inclusion is any material that is trapped inside a mineral during its formation. This can include other minerals or metals, plants, or animals. Insects found within amber gems are common examples of inclusions. In gemology, an inclusion is a characteristic enclosed within a gemstone, or reaching its surface from the interior. According to Hutton's law of inclusions, fragments included in a host rock are older than the host rock itself.
  • While inorganic by nature, it is shown with gems like Phosphophyllite and Yellow Diamond that Gems are susceptible to mental illness.
  • While completely different from any other existing lifeform, Gems share some similarities with endoliths and siphonophores in structure, with the feeding habits of photosynthesizing organisms.
  • Gems seem to have a concept of "growing up", as seen when Hemimorphite asks Phos who they want to be when they grow up, and how gems are split into seniors and juniors. How old a gem must be to be considered a senior is unknown, but likely to be once a gem is 1,000 years old or more.
  • Kongo-sensei is the only adult in appearance among the Gems.
  • Kongo-sensei/金剛先生 and Cinnabar/シンシャ are the only Gems who have Japanese names instead of English in the original Japanese version. Though note that "Shinsha" is written in katakana like all the other Gems' names, rather than its normal kanji (辰砂).
  • The Gems' uniforms are styled after mourning clothes, best seen in how their nightwear is pure white and folded opposite like you would the dead and more subtly in their winter uniform.
  • When Phos snuck onto the Moon, the Lunarians examine them and compare Phos to the "Apophyllites" that they used to bring in. There has been no other reference to a Gem called "Apophyllite", so it is unknown if this is referring to another Lustrous or simply the mineral itself.
  • Gems as a whole have poor eyesight, as they see by using the light that refracts within their bodies. Their eyesight is likely similar to someone with ocular albinism.