A Brooch is a piece of jewelry specifically designed to hold together a cloak. Often quite plain in period, some brooches were highly decorated, some reaching the status of high art.
Brooches are sometimes called Cloak Pins in the SCA. Although a misnomer, such "cloak pins" include annular, pennanular, and disk brooches.
Period-style brooches are hard to come by in mundane life, but are usually available (in greater or lesser variety) at SCA and other re-enactment events.