Moonstones, also known as muna riel in Elt, look like a crystal sphere that only light for certain people. These stones are made by Eletians, supposed to contain moonlight. To the user that can make the stone light it provides not only light but a sense of warmth.
Karigan uses a moonstone, that she was given by the Berry Sisters , to fight Shawdell the Eletian. In the book it is decribed as making a moon blade for Karigan to fight with, when the sword she had been using was destroyed.
Karigan also lights many places for herself when it was dark and she was miserable and hurting.
During the Blackveil event, it was discovered that the moonstone also activated the timepieces or the lamp.