[ seeing the sea only once is pretty hard to imagine for them. It implies the existence of a piece of land so big that there's a point in it where the ocean disappears from the horizon. Kongou implied that there used to be land like that in their world, too. Provided the things they've learned about their ancestors, they suppose that only makes sense - Vangeance probably lives on a world somewhat like that.

They were expecting him to ask that next question, though they were purposefully vague enough that he could have asked almost anything; it's the most important thing to learn about the way the gems live, after all. Cinnabar only buries the lede because they don't want to be pitied for that, too. ]

The moon. The ones who live there come during the day to try and take us away. Gems who go there don't come back.

[ their brow furrows a little bit as they say it, but they decide it's true enough. they're not sure what came back was really that gem anymore. ]

It's been going on longer than I've been alive. [ they pause, then fold their arms, closing their eyes with a small sigh. ] Before you ask, we don't know why. Not really.