Mohs Scale of mineral hardness: This characterises the scratch resistance of various materials by comparing the ability of a harder material to scratch a softer material. For example, talc measures 1, pure gold measures 3, hardened steel measures 7 , diamond measures 10. Alumina Ceramic sharpening stones manufactured by Spyderco, measure 9 on this scale. These are manufactured in USA, in a process which combines a bonding agent with alumina particles. Alumina particles are essentially synthetic saphires measuring 15 - 20 microns. The Spyderco sharpening stones are shaped and kiln-fired at 1649 degrees centigrade. Medium grit and fine grit stones are supplied with the Tri-Angle Sharpmaker 204MF. These have the advantage of lubricant-free sharpening, although after prolonged use will need cleaning to remove microscopic steel particles adhering to the stone. The sharpening effectiveness is gradually reduced as the stones become loaded with steel. They are easily cleaned by washing in fresh water and scrubbing with powdered abrasive and a scouring pad to remove steel residue from the open cell structure.