 As non-metals do not corrode they remain rust and corrosion free but they suffer from similar faults as stone and bone, being rather brittle and almost entirely inflexible.They are harder than metal knives and so more difficult to sharpen, and some ceramic knives may be as hard or harder than some sharpening stones.For example, synthetic sapphire is harder than natural sharpening stones and is as hard as alumina sharpening stones.