"Cubic Zirconium Corollary": The aforementioned mysterious girl will be wearing a pendant that will ultimately prove to be the key to either saving the world or destroying it.
How old is this? Old. When Plautus was writing Three's Company plays in the third century BCE, this was old. (Seriously. Three's Company. Go look it up.) And as it was good enough for a host of brilliant writers (including Victor Hugo or Somerset Maugham), I shouldn't go too far in dissing it here. The plot device is just too damn useful ... and frankly, I wouldn't be above it myself.
I don't have any fault that a pendant would ultimately, somehow, make the difference in the adventure - but does it have to be the only exceptional bauble mentioned in the entire campaign? Yes, this is a gem, and this is a necklace, and this is a pendant that is made of a substance you have never seen, with a weird and deeply compelling glow, that apparently serves no special purpose whatsoever.
Oh come on. We're not idiots.
If you will use this device (and I encourage its use), please try to remember that repeating the presence of the item over and over tends to destroy the 'surprise' element that is inherent in the device's inclusion. We don't need to be told every time the girl is mentioned that she is also wearing the pendant; it doesn't have to glow; it doesn't have to have any special characteristics at all. In fact, don't even mention the thing, at least not until Act III, when it should be accidently discovered and given some purpose that actually closes the player's mind on the subject.
NOT, I might add, having it said that "Oh, this thing? I don't know where it came from, or what it's for ... but I've had it since birth ..."
Why not just hang a neon sign around the girl's neck? Try instead, "It's a piece off my mother's ring - pretty, isn't it?" Say it with a really dumb voice and you might get that past the players. Even, "I bought it three months ago for two gold pieces - the vendor said it wasn't real." In other words, make the presence of the thing believable. You are not, after all, Charles Dickens.
Here's another thought. Make it a red herring. Make the damn thing actually have no value whatsoever. Then you can pump it up to your heart's delight, and laugh fit to kill watching the party trying to insert the thing into every crack and crevice from the top of the dungeon to the bottom. Until finally they show it to the big bad at the end, who takes it and says, "This? They sell these six for a copper in Xjjaytt!" before unconcernedly tossing it aside.
Sometimes there's no trouble with the cliche. It's all in how it's played.