It looks like it has some gum vein through it. Eucalypt burl from here in Australia sometimes has this through it and looks similar to your pen, so it could be a pieces of Eucalypt gum vein burl???
Don't have any idea myself. Maybe you can just tell everyone that it is _______ burl. Just make up some name.[}] Kirk[8D] BTW it IS a vey beautiful piece of wood!
Maybe when Bev gets done with it you could ship it to me and I could have my research department get right on that for you. (hehehehe) Very nice looking pen.