Bird's Egg

Eggs!

The item that is of no particular use to anyone but must be protected from harm despite hardly anything, if anything at all, negative happening if the protector fails their job. This also describes an actual bird's egg as the egg itself does not serve any purpose to the adult birds, but it must be guarded from predators and other forms of harm or else the undeveloped chick inside will have birth defects or die. A common Bird's Egg in fiction is a valuable diamond or piece of art (like a sculpture or painting) in a museum that is pretty and interesting, but also highly susceptible to thievery by the Burglar.