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.

Ring of clarity, which makes you immune to confusion when wearing it. If you happen to eat it, it will cure any temporary confusion you have.

Eating objects made of different metals should give different amounts of nutrition per unit of weight. Currently all metals give 1 nutrition per weight unit, but it could be that things such as copper or mithril would give less nutrition than iron, whereas gold could give more, for instance.

Eating cursed rings or amulets gives the opposite of the normal effect (likely stripping an intrinsic instead of gaining it).

Disenchanters derive additional nutrition from enchanted objects they eat akin to dNetHack disenchanters.

Eating a bag of holding confers temporary intrinsic hunger, and eating a bag of tricks confers temporary intrinsic aggravate monster.

Eating a wand of wishing polymorphs you into a djinni.

Eating artifacts lowers the game’s generated artifact count by 1 per artifact eaten, so that you can wish for more artifacts successfully.

You can eat cloaks of protection and invisibility to get those intrinsically. Eating a cloak of protection is equivalent to eating a +3 ring of protection.