Brass
I have used brass for many years as a medium for filling voids and cracks usually glued in place with thick CA. I occasionally use 5 minute epoxy. I also use it in very small pieces for inlay when A bowl or even a pen develops a crack. When finished, I still see the crack filled with brass or Turquoise or both as a crack. My customers see it as fine woodturning.
The brass is cut into small pieces with a small saw with a diamond blade or a carbide blade installed on my scroll saw. Like I said small pieces. When I have cut enough brass to make things worthwhile, I will take part of it and put it into a coffee grinder to turn some of it to small pieces, and some to dust.
The problem is the brass is harder that most any wood that you're going to work with, so you must learn to work smart and level it with a Dremel or small rotary tool. I like Ryobi.
Gently sand the pen on using your lathe and using sandpaper with a small block behind it. Keep working the wood and the brass until you have a symmetrical pen blank and put it together.