This knife sharpener made of synthetic ruby is by 20 times harder than hardened steel (20,000 Knoop hardness corresponding to roughly 200 Hardness in Rockwell units). The most unusual thing about this sharpener is the high efficiency (less time for sharpening) in combination with an extraordinary fine edge leading to high edge retention. Moreover, this sharpener has a higher life time than e.g, diamond sharpener as it is not a coated instrument but one of bulk material. This knife sharpener can be used either dry or wet. After use just clean it with a dishwashing sponge. Length of sharpening tube: 20 cm.