A ferrule is a ring of metal.A ferrule cap is the ring with one end closed off. It can be made from anything that can be forged or machined, and then welded or brazed. Make a ring from whatever material you choose,say mokume' for instance. Once the ring is shaped and fitted to the handle size, place a flat piece of mokume' on the end and braze/weld/solder it on.After trimming it up you have a ferrule cap. If you are just making a ferrule as a bolster,you don't need the end piece,just the ring.If the material is not conducive to welding/brazing then the end piece can be just physically attached tight to the ferrule with a tang nut or similar method.
Stacy