I think the example in the OP is already a winner. Go with that.
How about a few strands of optica fibra
Optical fiber - with the addition of a button cell and some minimal electronics you could make the tip glow in a suitably magical fashion
If I saw that growing on my property I’d move!
Surely you two must be exaggerating, it can’t be that [checks wikipedia] DEAR LORD! Kill it with fire!
How about a Herkimer diamond with a pocket of petroleum trapped inside? This one /www.etsy.com/listing/515007518/rear-petroleum-fluorescent-oil-diamond?ref=pla_similar_listing_top-2 looks as if there is some kind of spirit trapped there. And the oil fluoresces under UV light.
Or you could get an enhydro quartz or agate with water instead of oil.
Oh! It’s late in the season but perhaps you could get hold of some snapdragon seed pods. They look like little skulls!