Duskitine is a stone that reacts to starlight and is neither rare nor valuable.
A disk made of duskitine was found in Keefe's Neverseen cloak, which he gave to Sophie for protection while he was destroying part of Foxfire. Part of the Lodestar symbol was carved on the disk. If one were to enter a Neverseen Hideout without the corresponding duskitine disk, they would likely trigger an alarm of some sort.
Quotes[]
“It's made of a stone called duskitine, which is neither rare nor valuable. But it does react to starlight, which may be a clue to its purpose.”
—Mr. Forkle, in Lodestar