The brooch of shielding appears to be a piece of
silver or gold lewelry, 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.

(note)