Bone. Horns are keratin - think fingernails and hair. Antlers are shed and regrow every year. Horns grow for the lifetime of the animal. Surely a Viking would know the difference between a horn and an antler.
Thank you Brian for coming to my aid as if I needed it but it is much appreciated and thanks to you and John for finally realising the nicety of my fine attempt of showing "horn" and now I come to the point, horn. In Norwegian, we do not have the word antlers, but as Brian so elegantly puts it, we have horn. But Brian, not on the helmets! Yes the word horn sums up all kind of horns in the viking land. No other word for what a bull has, a reindeer or an elk. It is horn. And mind you, where do you think your word "horn" stems from. Like the word, man, glass, milk etc... The Vikings!