A
NetHack amulet that prevents your body from changing form. Specifically,
polymorph effects will not work. The amulet also confers protection against being
slimed by the feared
green slime -- this effect alone makes it worth wearing if you don't have an
amulet of life saving or an
amulet of reflection yet.
Eating the amulet has a small chance of bestowing permanent unchanging as an intrinsic; this can be a mixed blessing. Otherwise, there is no useful effect, though doing so may return you to human (or elven, dwarven, whatever) form.