NOTE: This article contains spoilers! Read with caution.

金剛先生 / Kongo-Sensei
First Appearance: Vol. 1, Ch. 1
Status: Active
Nickname: Sensei / 先生
Race: Gem
Age: Unknown
Hardness: Unknown
Family: The other gems
Occupation: Teacher and protector of the Gems
Father figure for the Gems
Teammate: None

Kongo-sensei (金剛先生, Kongō-sensei, Literally meaning "Adamant Teacher") is the head of the Gems, acting as the teacher, protector, and father figure. He teaches the Gems about the current world and its history, and decides the role suited for each of them. He has overwhelming offensive power and can disperse the Moon People in one blow. He becomes incredibly intimidating when angered.

Every time the Moon People appear, one Gem from a pair of patrol will ring their school's bell. He and some (or all) of the Gems will go to the scene and disperse the Moon People. Unlike the younger Gems, he can stay awake for a long time and doesn't require sleep, although he still rests during meditation periods, implying exhaustion. When angry, he can crack an entire building and his voice is able to shatter the weaker Gems in close range. Later, it's shown that he disperses the Moon People by cracking his arm and using very tiny fragments of himself to attack them.

From the Character Introduction:

  • Also known as 'sensei'.
  • Is considered the strongest and can be scary at times. It's rumored that his emotions actually shows on his face easily, but he just frowns to cover it up.


Kongo-sensei takes form of a bald, adult masculine man wearing a Buddhist monk kimono and robes. He always wears a pair of white gloves and usually sports a black cloak when going outside. He is often shown with a somber expression, frowning with his eyes closed.

He might be like the other Gems, who are covered in a white powder that imitates skin. If so, it is unknown what his true color is underneath the white powder. He's described as solid and heavy, as he is able to create trembles with his footsteps and crack pillars simply by dropping his head onto them when sleepy.


Normally, he is very quiet and stoic. When he speaks, he is straightforward and concise. He only rarely displays emotions, but this is rumored to be because he is deliberately hiding them and that his face would be very easy to read otherwise. On the rare occasions where he shows humor, he is still as deadpan as ever.

His composure usually only falters under two conditions: first, when Gems put themselves in harm's way carelessly, second, when he is confronted by matters from his past, such as the subject of humans. These cause him great alarm. When he is angered, he is capable of tremendous unintentional destruction, shattering weak Gems with the force of his voice alone or cracking stone floors by walking over them. These emotional expressions never last long and he very quickly reverts to his relatively expressionless face.

Otherwise, he is nonchalant and tolerant. He allows Gems without critical duties to do work on their own times, and is blasé about them prying into his past or secrets.

Kongo-sensei seems cautious using his power as to not shatter the Gems, unless he decides to scold them for making trouble or putting themselves in danger.

He cares for the Gems considerably and blames himself when they are injured or taken away by the Moon People. Kind and wise, he becomes sad when the younger Gems are sad. He only rarely demonstrates physical affection, such as hugging Antarcticite or patting other Gems on the head. For unknown reasons, he didn't tell the younger Gems about 'humans' and avoids any mentioning of them, instead vaguely referring to humans as an 'ancient species'.

He does not appear to sleep often, but his wakefulness takes a toll on him. He has a set period in a day where he goes under deep sleep-like meditation and becomes virtually impossible to wake. According to Jade, waking up sensei from this "nap" period is more of a struggle than fighting the Moon People, as demonstrated by the Gem shouting and even smacking at their sensei in a futile attempt to wake him. Despite that, it is implied that he can be awakened from this state, as he once chastised the Gems for not doing so when the school was attacked by the Moon People. During that same incident, he had drifted from meditation into sleep and starting dreaming, something which he considered reckless upon waking.

He is hinted to be very lonely.


  • Kongo-sensei's history prior the 6th meteor fall is unknown. Since he knew about it and the humans, it can be safely assumed that he was already alive when the humans existed.
  • He gave Phosphophyllite the job of making a natural history encyclopedia.
  • He undergoes one of his "meditation" periods, during which two Moon People sunspots appear, one of which attacks the school directly. The Moon People release the slug Ventricosus onto the school. After pacifying the slug, Phos introduced it to Kongo-sensei and he welcomed her. He asks Phos to accompany the slug as part of their role's work.
  • Phos asked him about Cinnabar's sadness and wasted potential on night duty, but Kongo-sensei apologizes to Phos and asks for forgiveness as he's still hadn't yet found a solution for Cinnabar and blames himself for the situation.
  • He became angry when Phos disobeyed him by going to the sea and ending up coming back without their legs. He relieved Phos of their first job as the composer of the natural history encyclopedia since it put Phos in constant danger and caused more trouble than it was worth.
  • When he asked Phos for a report about their experience under the sea, Phos mentioned "humans", causing Kongo-sensei to become alarmed and smash a table.
  • After Phos gained new legs, he assigned them to Amethyst, who patrols a spot where it's common for the Moon people to come.
  • Because Phos was unable to sleep, he agreed to let Phos team up with Antarcticite. Later he told Antarc to go to Shore of Nascency after Phos lost their arms, which leads to Antarc's abduction. He was trapped by the Moon People in front of the school. Kongo-sensei then guided and became Phos's teammate until winter was over.
  • He slept for a while and woke up to find "Shiro" inside the school. He asked "Shiro" about its arm and lied when Phos asked him whether he knew the creature. Phos then asked if the dispersed "Shiro" is returning to the moon, but he said that it seems already satisfied.
  • Kongo-sensei agreed to let Phos join him to fight the Moon People. Phos was surprised when Ghost Quartz suddenly joined them.
  • Kongo-sensei is a prayer machine built by humans in the past in order to pray for the souls of the dead in order to purify them. At some point prior to the story, his internal components became damaged and he consequently stopped purifying the souls of humans (The Moon People).
  • Kongo-sensei reveals to the remaining gems on earth that he is a different being with an origin different from their own, and further tells them that he had been trying to make a pure land for them but it only resulted into conflicts. He finalizes it by stating that they will have to continue without him, and that they should instead follow after one of their own, such as Phos.


The Gems like and trust him, and most of them obey him. Sometimes he pats them on their head and they love it. He also lets them crawl up onto him or he playfully picks them up.

The younger Gems have inkling that Kongo-sensei has a connection to the Moon People but according to Cinnabar they never discuss it openly and the Gems simply decided to trust him entirely. He also trusts them back as when Phos, the only one who can talk to Ventricosus' small form, told him that they could communicate with it and he believed them. He's happy when the Gems are happy, and sad when they're in trouble. The Gems that disobeyed him and ended up breaking themselves usually get yelled at, making them more broken.

He told Cinnabar that they're worth just being alive, when the latter was struggling with their body. Cinnabar themself dodged Phos' question and said they're still observing him.

He welcomed Ventricosus, the ruler of the shell, as he thinks the shell is also another kind of gem.

He let Phosphophyllite cling and hide underneath his robe. When Phos was lost in the sea, he was glad to get them back. He understands Phos' loss about Antarcticite and kindly guided them during the rest of winter. While Phos loves him too, Phos starts to doubt him after accidentally hear him calling the unusual Moon Person a name. When Phos found out that he has been using tiny pieces of himself to attack the Moon People, he asked Phos to keep it a secret.

For unknown reason, the Moon People in his dream are kind, hoping and worshiping him. The Moon People in reality somehow don't attack him when he's meditating. Instead, they peek at him from the window. But in winter, they worshiped him, then trapped him and made him unable to save Antarc in time. "Shiro" the Moon Person, on the other hand seemed like and obey him.

It is later revealed to Phos by Aechmea that the moon people are the dead souls of humans who were not purified by prayer, and thus stuck on the moon in a state of limbo. They wish for Kongo to relieve them of their eternity of suffering by praying for them.

Gem Info

The kanji used in Kongo (金剛) is derived from the Buddhist term Vajra, which means both "diamond" and "thunderbolt". A vajra is a symbol representing a force that is both indestructible and able to cut any substance (hence diamond), as well as unstoppable (hence thunderbolt). A vajra is often represented by a mace used by the Hindu god Indra.

"Adamant" seems to be a fan translation of Kongo. Adamant is a term used to refer to any especially hard, unyielding substance, whether composed of gemstone or some type of metal. Both adamant and diamond derive from the Greek word αδαμαστος (adamastos), meaning "untameable". Adamantite and adamantium (a metallic name derived from the Neo-Latin ending -ium) are also common variants. In the past, the term "adamant" has been used to refer to very hard materials such as diamonds. Today, adamant is often used in literature and other works of fiction to refer to a fictional substance that is unbreakable.

Kongo-sensei may be hinted to be a specific type of diamond called lonsdaleite based on the hexagonal pattern of his robes. This type of diamond is made of carbon atoms arranged in a hexagonal lattice. Theoretically, pure lonsdaleite is ~1.5 times harder than the usual cubic diamond.


  • 金剛/Kongō means 1. vajra (indestructible substance); diamond; adamantine 2. thunderbolt; Indra's weapon; Buddhist symbol of the indestructible truth.
  • The reason why he wears Buddhist monk attire may because one of his Japanese name's meaning, other than the fact he adapted one of 'human' religion/tradition.
  • Kongo-sensei so far is the only adult-like Gems shown.
  • Kongo-sensei/Kongō-sensei and Cinnabar/Shinsa are the only gems who use Japanese names instead English in original Japanese version.
  • He's voiced by Joji Nakata in the anime adaptation.