Russian guys make knives of sapphire. These knives cannot be discovered by any sort of metal-detector, they have no metal parts at all. Their blades are made from artificial sapphire, the same material that is used to make non-scratchable watches by leading Swiss brands. Handles are made of bone. Only diamonds exceed it in the hardness. It scratches glass without any difficulty and can be sharpened only with special diamond whetstone.