Akuyaku Reijou no Nariagari - Vol. 2 Ch. 7

[lasercats]

I'm betting its cinnabar or something.

 

[Grikath]

So maybe we should still bet on it being some sort of voodoo stone and that she'll be able to cure the empress through a ritualistic dance while being half naked and wearing body paint. It'll be funny to move on from that to becoming the crown princess.

Not sure about a Voodoo stone, but given the importance of (semi)precious gemstones in the story...
Could be the author set up the world in such a way that the attributed qualities of gemstones ( pick any tradition/system here, there's "a couple".... ) are actually quite real...
Which means that there's ways to combine effects/qualities to have detrimental effects as well as positive ones.

Not quite magic™, but close..

I'd be surprised if it's something like metal poisoning from the stones themselves, because while some gemstones are dangerous to work with ( cat's eye/tiger eye is a good one to get asbestosis, for instance...) , almost all rocks we consider gems are inert when worn, because all of them are silicon/aluminium based so the heavy metals that give them their colours are locked into the crystal matrix.

The setting could be poisonous, but that wouldn't be the actual gems...