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This article contains plot details from the latest books.
Lumenite is a material that was used for building Lumenaria (which is the origin of it's name) when the Ancient Councillors built it. It does not burn, even with Everblaze, so it is also used for bottling said substance when necessary.
However, in Lodestar, the Neverseen find a way around the no-burn rule and the confusing hallways and staircases, using Gethen to collapse the Lumenaria castle using outward channelling (a skill) during the Peace Summit, killing Mr. Forkle's twin (as revealed in Nightfall) and Brant while Councillor Terik lost his leg.
Fintan Pyren was once held in a Lumenite prison for a short period of time.