Pennannular brooches are common to many cultures. This particular style has been found in Viking silver hoards. I've seen Roman brooches of bronze. Convenient modern materials are copper (#6 wire, the size fastened to the pipes in your basement) and brass (3/32" brazing rod). You'll also need a hammer, a smooth block of metal, jewelers' pliers (preferably round-tipped), a file, and a gas flame. I'll illustrate using a propane torch, but I've also used gas kitchen stoves.
Use the pin to fasten a cloak at your right shoulder. This leaves your sword-arm free, just in case. Or take a square of cloth, a pin, and you have a shawl for those cold days.
Step 1: The body of the brooch
Take a piece of #6 copper, 5-1/4" long. Polish it, then bend into a circle with the ends about 1/4" apart.
Step 2: Beginning to form the pin
Heat one end of the brazing rod with a stove burner or propane torch. Hot hammer about 3/4" length flat, using the metal block for an anvil. Thin it out to about 3/16". Neaten and smooth the end. Polish.
Step 3: Forming the pin loop
Heat the flattened end to soften it. Curl into a loop which is a snug, but not tight, fit over the #6 copper.
Step 4: Embellishing the pin
Heat the brazing rod just where it passes over the copper ring. Leave the rod long enough that you can hold the cool end. Hot hammer the rod a bit wider, in a graceful leaflike shape. (This step is optional, but adds a bit of authenticity.)
Step 5: Finishing the pin
Trim the brazing rod, and file to a neat point. Make it just a bit rounded, for safety's sake. Polish.
Step 6: Joining body and pin
With the pin on the copper ring, widen the ends of the copper ring by cold hammering until the pin can no longer escape from the brooch. The metalwork is now finished, though a bit of final polishing can help.
Step 7: Using the brooch
To use the brooch to fasten two (or more) layers of fabric together:
1) Gather two layers of fabric. Push pin through.
2) Bring gap in ring down past pin.
3) Turn the ring underneath, so the pin is held solidly.