SOLVED. YA Historical Fantasy set in the eighteen hundreds, about a young woman who is taught how to use a special piece of glass in a necklace to identify demons though their disguises in the human by a gentleman with a scandalous reputation.

[Lillie]

I don't remember anything else about the book except for I said in the topic. Please help me find it if you can. It's driving me crazy on trying to find it.

[Jess]

Probably a long shot, but The Dark Days Club by Alison Goodman?

[Lillie]

Thank you that's the one I've been looking for.