I was taught that you could soak the bone in oil for a couple days (don't know what that does), then file it to near final shape. Burnish it very well, fire harden it the same way you fire harden wood, then do your final sharpening. I haven't tried it myself, but it was taught to me by some primitives that make bone arrowheads and knives. They claim they can get their edges nearly as sharp as flint, they just dull more quickly.