Cairngorm

Hardness: 7

Cairngorm is the inner crystal of Ghost Quartz, who has a multilayered structure. "Cairngorm", the name given to them by Kongou-sensei, is a variety of smoky quartz.

In a battle with the Lunarians, the outer layer of Ghost Quartz is stripped off and taken to the moon, leaving the inner Cairngorm behind. Afterwards, they take up Ghost's duties in their stead.

Ghost Quartz described Cairngorm as "rude, violent, and selfish", and said they would only listen to their old partner, Lapis Lazuli. But although Cairngorm was initially infuriated with Phos for "letting" Ghost get taken, they begin to act much kinder, and even become quite protective of them. The two gems become partners and are both assigned to winter duty.

However, Phos and Cairngorm are never able to go on winter duty together, because Phos gets attacked and their head gets taken to the moon. Afterwards, Cairngorm pleads with Kongou-sensei to use Lapis Lazuli's head to repair them. In fact, they are so desperate to repair Phos that they even offer up their own head as an option. During Phos's 102 year rest, Cairngorm tackles winter duty alone, and frequently visits the comatose Phos.

Later in the manga, Cairngorm reluctantly goes to the moon with Phos. Not long after, Prince Aechmea questions Cairngorm's extreme dedication to Phos and, upon examining the gem closely, discovers that their eyes still contain traces of Ghost Quartz which are affecting Cairngorm's behavior. After removing those traces, Cairngorm's behavior changes drastically: they remove their powder, begin dressing in feminine clothing, no longer seem to feel attachment towards Phos or Lapis Lazuli, and most notably, become extremely close with Aechmea. They have a very lovey-dovey relationship with eachother and soon get married, making Cairngorm the "princess" of the Lunarians.