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.