Sounds like the amulet simply needs tuning... (and I'm sure once you pick it and try to interact it will show exactly why it's proof ;)).
I don't see a problem with a single solutionbranch only being there if you invest the proper skill points. Just like some quests need animal talking.
Unlike those you can actually still complete them some different way, you can't say that there.
Some solution might need blacksmithing (to notice the strange markings of a sword for example), lorefinding (that's not a REGULAR McGuffin
or actually combatskills (my firemage notices how it's not a regular fire attack but something very special etc.).
And there's nothing wrong with that. Instead, it's definitely something I think a good RPG should have.
