He only knows that she's an alchemist, so that's all the ghosts would know too (they're totally doing this behind his back)! I checked back on what the helpful stones for each magic type are, and it looks like boji stones stabilise alchemists' potion combinations and prevent the dangerous ones from exploding and whatnot, so that might be a neat one.

As for the design, maybe a hair ornament or a ring..? Rinea's outfits don't really leave a neckline for a necklace. If Emet has other ideas the ghosts will surely listen!