An amulet of unchanging prevents you from changing your current form by polymorph or other means, and also protects you from being slimed; any sliming in progress is aborted. It does not prevent you turning to stone. Another exception is prayer as a lawful in undead form: you will rehumanize anyway.
When eaten, you are changed back to your natural form.
If you die while polymorphed and wearing an amulet of unchanging, you will not revert to your previous form, and will instead die.
However, if you "die" by polymorphing into a wood golem and getting rotted away by a brown pudding, you will resume your normal form. If you are polymorphed and decline to die in wizard mode, you will also rehumanize.
Amulets comprise 1% of all randomly-generated items in the main dungeon, 1% in containers, and 4% in Gehennom. There is a 4.5% chance that a randomly-generated amulet will be an amulet of unchanging, making it the least generated amulet.
Because uncontrolled polymorphing and being slimed are both potentially dangerous if the amulet isn't unchanging, and losing all HP while polymorphed is lethal if it IS unchanging, this is a relatively difficult amulet to identify safely.
The easiest way is probably to eat a mimic corpse while wearing the unidentified amulet. If you don't mimic gold/an orange, the amulet is unchanging. If you do, the amulet isn't unchanging, you won't break or fall out of your armor, and if a monster tries to pick you up, you return to normal form immediately.
If you're following the vegetarian or vegan conducts, an alternative would be to polymorph yourself while wearing the amulet and a ring of polymorph control, specifying a safe form if you do polymorph. Note that if you intend to use a polytrap for this purpose, you must remove any source of magic resistance as well.
If discovered early, this can be a good substitute for magic resistance in the early game, where polymorph traps are a big threat.
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