__Hardness:__ 10

The oldest gem, who mostly does patrol duty. Yellow is portrayed as cheerful yet mature, but struggles with low self-esteem and feels responsible for all of their siblings.

~!They are one of the gems who accompanies Phos to the moon.!~

~!During their time on the moon, Yellow gradually becomes more and more mentally unstable, and eventually even begins to suffer from delusions, thinking of themselves as a Lunarian and becoming distressed whenever they realize they're really a gem.!~

In a world inhabited by crystalline lifeforms called The Lustrous, every unique gem must fight for their way of life against the threat of lunarians who would turn them into decorations. Phosphophyllite, the most fragile and brittle of gems, longs to join the battle. When Phos is instead assigned to complete a natural history of their world, it sounds like a dull and pointless task. But this new job brings Phos into contact with Cinnabar, a gem forced to live in isolation. Can Phos’s seemingly mundane assignment lead both Phos and Cinnabar to the fulfillment they desire?
 
(Source: Kodansha USA)
 
Note: Nominated for the 8th Manga Taisho in 2015.

In the mysterious future, crystalline organisms called Gems inhabit a world that has been destroyed by six meteors. Each Gem is assigned a role in order to fight against the Lunarians, a species who attacks them in order to shatter their bodies and use them as decorations. 
 
Phosphophyllite, also known as Phos, is a young and fragile Gem who dreams of helping their friends in the war effort. Instead, they are told to compile an encyclopedia because of their delicate condition. After begrudgingly embarking on this task, Phos meets Cinnabar, an intelligent gem who has been relegated to patrolling the isolated island at night because of the corrosive poison their body creates. After seeing how unhappy Cinnabar is, Phos decides to find a role that both of the rejected Gems can enjoy. Houseki no Kuni follows Phos' efforts to be useful and protect their fellow Gems.


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The Gems fight against the Lunarians, beings who attack them and use their shattered bodies as decorations. Each Gem is assigned a role, and Phos, the youngest, is assigned to create an encyclopedia of natural history.
 
Note: A promotional video that was released through YouTube in commemoration of the publication of the manga's first volume. 
(Source: HIDIVE)

Yellow Diamond