0xp Even though I agree with Strolen, that the idea is something I would use as an annoyance rather than a plot itself, it's still a pretty good idea.

You have some seemingly innocuous incident of finding some insect riddled corpse in the den of some man eating monster or something(take your pick of man eating monsters), so that the necklace doesn't appear to be the cause of the insect horde. The sort of thing that would appear to be a random encounter, only not really random. After defeating the random encounter, they search the body, oh my, what's this, a fancy necklace, thank you very much mr dead body!

Then you continue them down the path of an adventure. At some point in the adventure, typically a fairly critical but non-mortal point, a spell fails because the wizard couldn't concentrate enough to learn his spell correctly. Then it happens again, after another night, soon, he's not able to learn spells at all, because of all the bugs waking him up over and over again in the night. Same thing could happen for a priest learning spells. Maybe the warriors get tired from lack of sleep and can't hit effectively.

MAYBE they get so tired that paranoia steps in. Everyone is out to get you, even your own comrades. Well, maybe, but I find plots like that tend to be annoying for the players in a cooperative group. Just a thought, I suppose.

Of course, you need to have it happen that the necklace is not suspected for a while to get much play out of it.