A new race whose primary feature is that it eats minerals: gets nutrition mainly from rocks, may be able to eat metals, and gains limited spell-like abilities based on the last precious gem they consumed. A couple non-comprehensive examples:

• Eating a ruby grants the power to cast fireballs.
• Eating a jet stone grants the power to make clouds of darkness.

The abilities could be triggered using #monster, and would probably have a finite pool of uses, after which they can no longer be used. Eating another gem before this pool runs out would swap in new abilities and either give a fresh pool of uses or add to the existing one.

If you’re polyselfed to a lithivore, worthless glass tastes terrible but valuable gems taste delicious. Each gray stone gives a different YAFM: “Yabba-dabba delicious” for flint, “That was a heavy meal” for loadstone, “The stone’s texture is delightful/distasteful” for touchstone, ??? for luckstone.

Lithivorous monsters can gain intrinsic speed by eating a dilithium crystal.