What It Is
The bone shield is the necromancer's work, and the necromancer is the only one who makes it, because the necromancer is the only one who is not afraid of what it is. It is the shape of a round shield grown from the rib or the shoulder of the dead, and the dead give it a property no living wood has: the dead do not rust, and the dead hold a word. Bone is hard the way cold is hard — it does not flex, it does not rot, it does not drink — and an enchantment needs a ground that will not move.
A journeyman bone shield is one bone, cut round, dressed in Hide, and set with a ring of Iron. It is sturdy before the working, and the working is what makes it a story. A master bone shield carries the working in the bone itself, under the dressing, where it cannot be scraped off. A masterwork bone shield is a story event, and the story has not written it yet.
The Making
The bone is chosen, not taken. The necromancer works the ward into the bone while the bone still remembers its own shape — the curve of the rib, the grain of the shoulder — because the working follows the grain, and a bone cut against its grain holds the word the way a nail holds a splinter: loosely, and until the rain. The finished shield is dressed in hide and ringed with iron, and the iron is not for looks. The iron is the border of the ward, and the ward is the one who is hit.
"Iron rusts and the rust eats the word. Bone holds, and the word holds with it." — Wreckreate