Antarcticite

アンタークチサイト

They oversee winter. They act like a soldier and like an only child. Their hardness is 3.
Antarcticite is liquid during the other seasons, but they crystallize during the winter. They also grow stronger as temperatures drop.
While the other Gems are hibernating, they battle the Lunarians alongside Kongou-sensei. Since there are only a few sunny days, it is easy to predict when they will attack. Their duties also include cutting ice floes, as the sound of the floes scraping against each other might disturb the sleeping Gems.

Spoiler