Irwin: "The fingerboard is made of gaboon ebony and has twenty-four frets. It is bound on each side with four laminations of maple, purpleheart and ebony."
More specifically, the binding (moving from the inner to the outer edge) is maple, purpleheart, maple and ebony. The maple/purpleheart/maple segment is roughly the same width as the outer ebony segment, perhaps a tiny bit wider.