I have recently bought myself a miniature for the Xorn, a D&D monster which eats precious metals or minerals.
Aside from that, I was recently reading Green Devil Face #4 and one of the articles ('Wand of the Weird') had a table for random effects a magic wand could do. One of them was the following:
"The subject’s eyes turn into gems worth
5,000gp each. The eyes function as normal
as long as they remain in the subject’s
Now... You can also insert that effect into a magic trap, a spell, a mutation, it doesn't matter... What matters is that to someone who suddenly has gems for eyes, the Xorn has gone from being a nuisance to being a terrifying monster who wants to eat his eyes!!!