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 Spyderco Sharpmaker 204MF . Note that after prolonged use, rods will need cleaning to remove microscopic steel particles adhering to the stone. The sharpening effectiveness is restored by washing in fresh water and scrubbing with powdered abrasive and a scouring pad to remove steel residue from the open cell structure. Spyderco Sharpmaker Medium Grit Rod 204M1, Spyderco Sharpmaker Fine Grit Rod 204F1, Spyderco Sharpmaker Ultra Fine Grit Rod 204UF1 may be purchased separately. Also available: Spyderco Sharpmaker 204D Diamond Rod Pair

Spyderco Sharpmaker 204MF Knife Sharpener Spyderco 204 £52.99	Spyderco Diamond Rods Tri Angle Sharpmaker Diamond Rod Pair 204D £47.99
Spyderco Diamond Rod Tri Angle Sharpmaker Diamond Rod Single Rod £23.99	Spyderco Ultra Fine Rod Tri Angle Sharpmaker Single Rod 204UF1 £17.99
Spyderco Fine Rod Tri Angle Sharpmaker Fine Rod Single Rod 204F1 £15.99	Spyderco Medium Rod Tri Angle Sharpmaker Medium Single Rod 204M1 £15.99