Celestite

Celestite is a Gem that's been around for a bit. They were around when Pink Diamond was still on Homeworld. They were originally White Diamond's Celestite, a type of Gem that assists with creating maps of the stars (that ultimately lead to finding colonies to mine), but were considered defective. Their defectiveness isn't physical, and only lead to them struggling with concentrating and having a loyalty issue. There was an attempt to discipline these issues out of Celestite, but it didn't work. Celesite found out that the Diamonds were using Celest's maps to destroy other planets and ran away as they didn't want to be a part of that.