The brooch of shielding appears to be a piece of silver or gold jewelry, usually (90%) without gems inset, which is meant to fasten a cloak or cape. 

It has the property, however, to absorb magic missiles of the sort generated by spell, wand, or other magic device. 

A brooch can absorb up to 101 hit points of magic missile damage before it melts and becomes useless. 

Its use can normally be determined only by means of a detect magic spell and then experimentation.