Star Wars Jedi: Fallen Order - Cal finds his Kyber Crystal on Ilum.

Cal Kestis, travels to the ice planet of Ilum, long ago, the immense Kyber Crystal caverns, were turned into a Jedi Temple and it was considered a sacred right of passage, for every young padawan, to journey to Ilum and retrieve a crystal to construct their own light saber.
Many years after the fall of the Galactic Empire and the Sith, Ilum was occupied by The First Order and transformed into Star Killer Base.
It is thought, that The First Order chose Ilum for the bases, because of its high concentration of Kyber Crystals, which were an essential component, in their planet destroying super weapon.