Cairngorm

Cairngorm used to be the inner shell of Ghost Quartz, who has a multilayered structure and thus two personalities. After Ghost Quartz was taken to the moon, Cairngorm became active in their stead.

Ghost Quartz described them as "rude, violent, selfish" as well as annoying. Ghost claims that Cairngorm was in control of their body when they recklessly climbed onto a sunspot cloud and almost got taken by the Moon People. They seem to be very tough and strict. When they lost Ghost Quartz, they were infuriated and blamed it on Phosphophyllite, punching them several times. At first, they acted harshly towards Phos. But they began to soften towards the other Gems after Phos had a breakdown and had confused Cairngorm for Antarcticite.

Later, Cairngorm has begun to act much kinder and gentle towards Phos, and has become rather protective of them. When Phos lost their head to the Lunarians, Cairngorm even offered their former partner, Lapis Lazuli's head, to Phos. When Kongou seemed unsure of this, Cairngorm even began to offer their own head to Phos.

After Aechmea reveals that reviving the lower hardness Gems would not be feasible, Cairngorm offered to have a synthetic outer layer attached to disguise themselves as Antarcticite and calm Phos' trauma. Aechmea questioned Cairngorm's dedication to Phos and examined the Gem closely, discovery that their eyes still contained traces of Ghost Quartz which was affecting Cairngorm's behavior. At that revelation, Cairngorm felt emotional distress, not wanting to be trapped by Ghost again, and began to break apart, basically agreeing to let Aechmea remove Ghost from their eyes.
(Source: Houseki no Kuni Wiki)